Basic Science Considerations in Primary Total Hip Replacement Arthroplasty PDF

The Open Orthopaedics Journal, 2010, 4, 169-180 169 Open Access Basic Science Considerations in Primary Total Hip Replacement Arthroplas...

The Open Orthopaedics Journal, 2010, 4, 169-180 169 Open Access Basic Science Considerations in Primary Total Hip Replacement Arthroplasty Saqeb B. Mirza*,1, Douglas G. Dunlop2, Sukhmeet S. Panesar3, Syed G. Naqvi4, Shafat Gangoo5 and Saif Salih6 1 Trauma and Orthopaedics, Southampton University Hospitals NHS Trust, UK 2 Southampton University Hospitals NHS Trust, UK 3 National Patient Safety Agency, UK 4 Walsall Hospitals NHS Trust, Birmingham, UK 5 Weston General Hospital, UK 6 Avon Orthopaedic Centre, Bristol, UK Abstract: Total Hip Replacement is one of the most common operations performed in the developed world today. An increasingly ageing population means that the numbers of people undergoing this operation is set to rise. There are a numerous number of prosthesis on the market and it is often difficult to choose between them. It is therefore necessary to have a good understanding of the basic scientific principles in Total Hip Replacement and the evidence base underpinning them. This paper reviews the relevant anatomical and biomechanical principles in THA. It goes on to elaborate on the structural properties of materials used in modern implants and looks at the evidence base for different types of fixation including cemented and uncemented components. Modern bearing surfaces are discussed in addition to the scientific basis of various surface engineering modifications in THA prostheses. The basic science considerations in component alignment and abductor tension are also discussed. A brief discussion on modular and custom designs of THR is also included. This article reviews basic science concepts and the rationale underpinning the use of the femoral and acetabular component in total hip replacement. Keywords: Hip replacement, arthroplasty. 1. INTRODUCTION This article aims to provide a basic scientific understanding of the rationale behind the use of different THR is one of the most common operations performed varieties of acetabular and femoral component for THR used on the NHS. About 40000 primary THRs are performed in in different situations. It is by no means intended to provide NHS hospitals in England with about 4000 revision hard and fast rules on their use. procedures being performed. An increasingly ageing population means that absolute numbers of people with a 2. RELEVANT JOINT ANATOMY IN THA predilection for osteoarthritis is set to rise. It is estimated that The hip is a ball and socket joint in which stability is THRs will increase by 40% over the next 30 years due to obtained by the bony configuration combined with a demographic change and it is projected that the highest complex system of muscles and ligaments around the joint. rate of increase will be in the middle aged and over 85s. The femoral head diameter averages about 46mm. Two There are differing guidelines in terms of indications for THR among different a countries, which makes direct critical angular relationships of the femoral neck with the shaft include the neck shaft angle which averages 130 comparison of data difficult to make. Although being a very degrees and the femoral anteversion angle which averages 12 cost-effective operation (THR cost-utility analysis estimates degrees. Femoral neck version is the angle of the femoral that the cost per QALY is £700, compared to £3000 for a neck with the intercondylar plane. The hip joint contribution kidney transplant), the cost to the NHS is still substantial and to lower limb length is the vertical distance from the femoral in 1992 was estimated at 231.3 million pounds. head centre to the lesser trochanter. Femoral offset is the There are numerous types of femoral components made horizontal distance from the midline of the longitudinal axis by different companies and many have been introduced into of the femur and the centre of rotation of the femoral head the market within the last 5-10 years and hence long-term (Fig. 1). Individual variations and conditions that affect head follow-up data is not available on a large proportion of them. neck angle and femoral anteversion lead to changes in femoral offset and hip joint contribution to limb length. For example patients with hip dyplasia may have coxa valga *Address correspondence to this author at the Trauma and Orthopaedics, and increased femoral anteversion, with resultant decrease in Southampton University Hospitals NHS Trust, 16 Catherine’s Gate, offset and increase in limb length while in coxa vara, the Haverfordwest, Pembs SA61 1NB, UK; E-mail: [email protected] femoral neck angle is reduced, leading to greater offset and 1874-3250/10 2010 Bentham Open 170 The Open Orthopaedics Journal, 2010, Volume 4 Mirza et al. tendency to shortening. These conditions also pose a challenge in THA and need careful preoperative consideration. Femoral head diameter is normally at least 1.2 times the neck diameter. Anterior impingement may result with lesser ratios. Acetabular anteversion is the amount of forward flexion of the acetabulum as measured from Coronal Tilt lateral to medial with reference to the sagittal plane and angle averages about 15 degrees. The acetabular abduction angle is the relationship of the line extending from the anteromedial and superolateral extents of the acetabulum with the horizontal. The acetabulum averages 15 degrees of anteversion and 45 degrees of abduction (Fig. 2). The natural curve of the femur in an antero posterior direction is about 4 degrees. It is also important to note the three types of Anteversion femoral shape based on metaphyseal-diaphyseal anatomy. angle Dorr type A femurs have wide metaphyses and narrow diaphyses, type B have a smooth metaphyseal-diaphyseal transition and type C do not have much difference in the sizes of these two regions. a Fig. (2). Schematic diagram showing acetabular coronal tilt and anteversion angles. c upward force is generated at an angle by the resultant of the abductors, and the body weight generates a force that acts vertically downward (Fig. 3). The ratio of the body-weight lever arm to the abductor lever arm is approximately 2.5:1.0. b The abductors provide two thirds of the hip joint force parallel to the long axis of the femur [10, 11]. The resultant a - Lateral offset force across the hip joint in the frontal plane makes an angle b - Neck length of 15-25 degrees to the long axis of the femur, producing axial compression, a varus moment and a medial-to-lateral c - Vertical Height force. It can be seen that with increasing offset and/or cup medialization that resulting joint reaction force (JRF) can be reduced, which is one of the surgical principles of THA advocated by Sir John Charnley. In the sagittal plane, a torsion force is created by the anteroposterior component of the resultant forces [13, 14]. This is equivalent to an axial torque on the femoral component similar to a wheel brace used to tighten nuts on a car wheel. FEA also shows high Fig. (1). Schematic diagram showing parts of a standard femoral stresses at the bone-implant interfaces , maximum in the component for THA. proximo-medial and the distal-lateral regions, though their 3. BIOMECHANICAL CONSIDERATIONS magnitudes may vary with particular prosthetic designs and materials. This explains why these areas are often Kinematically, the three axes of hip joint movement are flexion-extension, abduction-adduction and internal-external Ab - Abductor force Ab JRF rotation. Finite element modelling (FEA), a computer- A - Abductor moment arm generated method of analysing stresses across an artificial joint surface or predicted stresses across a material [6, 7] can B - Moment arm of body weight be used to simulate changes across the artificial joint as part A B JRF - Joint reaction force of surgical planning. It is estimated that the hip joint has to W - Body weight withstand each year, with cyclical loading, an equivalent of 3-6 times body weight due to contraction of the abductors , and peak loading 106 steps of 7-8 times body weight is seen in sporting activities. W The system can be simplified to consist of a lever arm where the hip joint is the pivot and forces on the femoral Fig. (3). Schematic diagram showing the Joint Reaction Force head are equal and opposite to those on the acetabulum. An generated by the abductor lever arm. Basic Science Considerations in Primary Total Hip Replacement Arthroplasty The Open Orthopaedics Journal, 2010, Volume 4 171 preferentially worn in loose femoral components, seen at the implant can be re-established in areas of disruption. time of surgery as excessive wear. Local contact stresses as Uncemented components may be surface-engineered in two well as shear stresses play a role in uncemented versions of ways to encourage bony interlock. Porous coating is where prosthesis. the implant surface has been treated to have many microscopic pores of varying depth, into which bone may 4. MATERIAL AND STRUCTURAL PROPERTIES grow. Grit blasting bombards the implant with microscopic Young’s modulus of a material is defined as stress particles that create indentations on the implant surface onto divided by strain where stress is given by force per unit area which bone can grow. and strain by change in length as a function of original 6. BEARING SURFACES length. It is a property not governed by shape as it has no units, but given similar shaped items, defines the different Traditionally, two types of bearing surfaces have been sensations of flexibility when made of materials with utilised in THA. Hard-on-soft bearings have included different young’s moduli. In artificial materials the elastic couplings where the acetabular liner has been polyethylene modulus is often linear, while biological materials including (PE) and the femoral heads have been metal, usually cobalt- bone usually display viscoelasticity, whereby it’s material chrome, or ceramic. Recent advances in these articulations properties are time-dependant and depend on the rate of have included processes to improve surface hardness and loading. Viscoelastic materials often display hysteresis resistance to adhesive and abrasive wear of the PE where loading and unloading curves do not exactly overlap, component and to reduce the rate of aseptic loosening and and energy is lost within the material as internal friction. osteolysis. Hard-on-hard bearing surfaces have included ceramic-on-ceramic (COC) or metal-on-metal (MOM) Bone is an anisotropic material. This means that it’s surfaces and these have gained popularity because of mechanical properties are greater in one direction than significantly lower wear rates than hard-on-soft bearings. another, due to alignment of collagen fibrils and osteons. Bone also exhibits creep, whereby when subjected to a 7. MECHANISMS OF WEAR IN THA BEARINGS constant load for an extended period of time, will continue to deform at decreasing rates. The converse is true when Three main types of wear have been recognized in THA subjected to a constant deformation rate and is termed stress couplings. Abrasive wear is caused by two surfaces of relaxation. Strain rate is also important, whereby rapid different hardness articulating against each other causing application of even a modest force can lead to fracture, particles to be removed from the less abrasive substance with compared to a higher force at a lower rate. Bone and less surface hardness. Adhesive wear is most commonly implants used for THR have different material and structural caused in PE articulations where PE particles are sheared off properties and together, form composite beam structures that and deposited within the joint space, stimulating an function together. osteolytic reaction. Third-body wear is when particles entrapped between two contacting and articulating surfaces 5. BASIC TYPES OF TOTAL HIP ARTHROPLASTY causes wear of the softer articulating surface. Types of arthroplasty are commonly described with 8. ASEPTIC LOOSENING AND OSTEOLYSIS reference to modes of fixation into cemented and uncemented types. It is currently thought that wear particles generated from the acetabular polyethylene are the main inducers of the 5.1. Cemented Components macrophage and histiocytic response that leads to osteolysis. Aseptic loosening is thought to be initiated by a Polymethylmetacrylate (PMMA) is the material combination of mechanical factors including cyclical loading commonly used for prosthetic stabilisation. This is based on and impingement of the neck on the acetabular margin. the concept that cement interdigitates within bone and that Gruen described four mechanisms of cemented PMMA is stronger in compression than in tension. Cement acts as grout and therefore there is no true adhesive component failure. The first is pistoning where the stem and/or cement subside into the femur. The second is the bond between the prosthesis and bone. During initial medial midstem pivot where a varus positioned stem fails at cementing, mechanical, vascular, thermal and chemical the proximo-medial and distolateral areas. Calcar pivoting is trauma play a role in disturbing normal bone function. The the third where the distal aspect of the stem can shift within endosteal blood supply is damaged and endosteal necrosis the distal cement mantle. Loss of cement mantle proximally occurs up to a depth of 500 micrometres. Over the ensuing months this blood supply is re-established as fibrovascular and fatigue failure of the proximal stem due to repetitive loading at a compromised cement-implant interface is termed granulation tissue and a new interface between cement and cantilever bending. Loosening has been demonstrated even bone is generated. before the fibrous membrane forms and is possible evidence 5.2. Uncemented Components that the initiating factors are mechanical. Biological factors then cause further loosening, leading to progressive This is thought to represent a truly biologic method of osteolysis, undermining the remaining bony support, which implantation in that the coated surface of the metal implant when severe, can lead to macroscopic rather than encourages in-growth or on-growth of bone onto the implant. microscopic loosening, with subsidence and risk of Cortical bone grows into the porous channels within the periprosthetic fracture. In the articulation, adhesive and metal implant to create a rigid interface. The philosophy abrasive wear generates wear particles from the softer behind uncemented THR is the establishment and material, typically PE. Biologically active wear particles maintenance of a rigid bone-implant interface that has range from 0.1-10 micrometres with those in the range of remodelling potential such that bony intercalation into the 172 The Open Orthopaedics Journal, 2010, Volume 4 Mirza et al. 0.1-0.5 micrometres being the most potent. Phagocytosis of localized endosteal osteolysis. Varus stem positioning these particles by macrophages cause release of cytokines results in a thin cement mantle in the proximal medial and and prostaglandins into bone stimulating osteoclastic activity distal lateral regions of the femur where the cement mantle and causing osteolysis. Release of oxide free radicals experiences the highest stress levels [33, 37] and can lead to and hydrogen peroxide also cause bone resorption. cracking and failure. There is a variable relationship between Inflammatory mediators including interleukin-1 (IL-1), the medial aspect of the greater trochanter and the centre of interleukin-6 (IL-6), tumour necrosis factor alpha (TNF-
), the femoral shaft and errors in positioning of the femoral prostaglandin-E2 (PGE2) and tumour necrosis factor-beta component may result secondary to an inappropriate entry (TNF-) stimulate osteoclasts. Monocyte colony stimulating point. The piriform fossa serves as a reliable entry point for factors (M-CSF) and granulocyte colony stimulating factors femoral preparation as its position remains relatively (G-CSF) stimulate osteoclast precursors. Interleukin-1 also constant. Thus we use this point as our starting point for inhibits osteoblast function. Other mediators such as femoral preparation in many of our THAs. collagenase and metalloproteinases cause direct bone A reduction in longitudinal compressive stresses has been osteolysis [23, 24]. Huge numbers of wear particles have demonstrated especially proximomedially by about 20-30% been demonstrated from prosthetic joints with an [38, 39], particularly when stems with a high elastic modulus estimated 38000 particles per step for a 22mm prosthetic are used, causing stress shielding. A mismatch between head. Although several studies have suggested a critical relative stiffness of the implant and the host skeleton wear volume related to the occurrence of osteolysis, other determines the severity of stress shielding. Materials with factors that have been shown to influence the extent and higher elastic moduli cause more stress shielding than those severity of the osteolytic reaction include total number of with lower ones, as do larger diameter stems and implants particles, size and morphology of particles with irregular placed more eccentrically than centrally. Osteopaenia shapes being more immunologically active than spheres [23, and resorption of bone away from cement over many years 24]. may increase interface stresses thereby contributing to 9. CONCEPT OF EFFECTIVE JOINT SPACE (EJS) loosening. Many stems are currently made from titanium alloy to exploit the beneficial effect of having an This concept describes the entire volumetric area within elastic modulus closer to that of bone and hence lower the hip arthroplasty construct, that can theoretically be bending stiffness. This transmits load to the proximo-medial infiltrated with wear particles and macrophages, potentially bone more efficiently to avoid stress shielding. causing osteolysis. For example, the use of bone screws for the fixation of metal shells increases the effective joint Smooth contours and sectional shapes that do not twist space. An understanding of this concept is important as any within the cement mantle are usually utilized to increase reduction in the EJS could, in theory, reduce the area that can rotational stability. Sharp edges on implants are avoided as potentially undergo osteolysis. they are a cause of stress risors within cement [42, 43]. Stems with rounded medial borders reduce stress 10. DISCUSSION concentration at the medial cement mantle where failure can 10.1. Cement Fixation occur. Proximo-lateral projections increase the connection between the stem and cement, increase Third generation cementing techniques that include compressive stresses and reduce tensile stresses within this vacuum preparation of cement to reduce porosity and region. Stem designs that are broader laterally than medially increase cement strength, distal plugging of the canal [26,27] diffuse compressive stresses medially and increase torsional are now commonplace and we certainly use them in our and bending rigidity. practice even though the evidence for some of them may not be fool proof. Preparation of the femoral canal by A femoral stem functions either as a composite beam or a pulsatile lavage and brushing and the use of a cement gun taper slip model with both having different mechanisms of and cement restrictors significantly improves the quality of load transfer. In the composite beam model, the stem is cement-bone interdigitation. We also use sponges considered a rod within two tubes, cement and bone, and impregnated with adrenaline or hydrogen peroxide to ensure depends on strong bonding between both interfaces to form a adequate haemostasis at the bone-cement interface just prior stable construct from three materials with different to cementing. Using cement centralizers both proximally and mechanical properties (metal, cement and bone). Load is distally where applicable ensures a uniform cement mantle transmitted via the femoral head and stem to it’s tip, of 2-4mm [30, 31] to prevent loosening and fractures in the bypassing the proximal femur and thereon to the bone cement mantle in regions of increased metal-bone proximity. cement and subsequently to host bone. A loaded, The size of the cement mantle is controversial. A polished taper stem on the other hand, must be able to move cement mantle of less than 2mm has been shown to within it’s cement mantle to function as a loaded taper. The demonstrate increased cracking and breakdown [30, 32, 33] load is transmitted from the prosthetic head and forces the and therefore our opinion is that it is best to maintain a taper to subside within the cement mantle, creating radial greater than 2mm cement mantle circumferentially, but compressive forces within cement and hoop stresses within particularly in the proximomedial part of the femur. In our bone, thus minimizing proximal stress shielding. Thus practice we try and establish a 4mm cement mantle in this these two biomechanical systems require different region as this is an area of increased stresses. Avoiding gaps prosthesis-cement interfaces, a perfect stem-cement bond for in the cement mantle is of paramount importance to prevent the composite beam system but no bond between the stem areas of direct contact between the prosthesis and bone that and cement in the taper slip design. lead to stress risors and early failure [34, 35] and to prevent Basic Science Considerations in Primary Total Hip Replacement Arthroplasty The Open Orthopaedics Journal, 2010, Volume 4 173 For our cemented implants, we generally use smooth 10.2. Uncemented Components polished stems that allow physiologic subsidence due to Many studies have demonstrated increased failure rates viscoelastic properties of cement during cyclical of cemented components in younger more active patients. loading. The quality of the cement mantle in Gruen zone 7 Other studies have demonstrated improved performance and ensures most of the load is transmitted to the proximal one third of the femur. Rough surfaces on cemented stems decreased loosening in patients who have had uncemented components in the short to medium term. This, have not been successful due to increase in shear and tensile combined with the theoretical advantage of having a stresses at the interface, causing progressive debonding and potential life-long bond between the implant and host bone interface micromotion [41, 49]. Cement fixation does create due to remodelling potential at the interface, has led to an two separate interfaces. These include the cement-bone and increase in uncemented THAs. the cement-implant interface, with no remodelling potential at either one of them. In cemented prosthesis, a strong 10.2.1. Material Considerations adjacent cortex is required for success to provide sufficient Cobalt-chrome alloys are extensively used for cement interdigitation for long term fixation. manufacture of implants. They have a high ultimate strength, Mechanical lock is central for cement fixation in any are biocompatible, easily workable and relatively resistant to situation, and this becomes a particular problem where large corrosion. However, they do have a high modulus of cavitory defects exist and a smooth bony surface usually prevents cement interdigitation. The success rate of elasticity and thus a higher bending stiffness and could contribute to stress shielding. Long-term effects of metallosis cemented revision THRs has thus been poor [50-52]. Indeed arising from cobalt-chrome are unknown and is the subject Katz demonstrated a 26% failure rate with the use of of research in many labs. cemented revision prosthesis. Another study looking at the shear stresses at the bone-cement interface demonstrated Stainless steel is still used for the manufacture of some this figure in the femur to be only 20.6% of primary strength femoral implants despite it’s tendency to corrosion and lower after a single cemented revision and only 6-8% after a fatigue strength. Some of the most successful implants are second cemented revision. In Dorr type C femurs or in made from stainless steel due to it’s relatively high strength, patients who have poor bone quality in whom uncemented cost and easy workability. components are not likely to do well, we use cemented Titanium-alloy has been increasingly used for femoral implants. PMMA-coated femoral implants have been shown stem manufacture due to it’s superior biocompatibility and to improve the interface between the stem and cement relatively lower elastic modulus, thus reducing the problem mantle, but have not resulted in lower rates of loosening [55, of stress-shielding. It is corrosion resistant due to the 56]. Overall, cemented components have demonstrated formation of a protective titanium-oxide layer on the surface excellent long term survivorship with revision rates of 0-5% by spontaneous passivation. It’s surface is also easily at more than10 year follow-up [57, 58]. In our experience modified to enhance osseointegration. However, it is and according to the literature however, cemented acetabular softer than other metals and cannot be used for femoral components are not routinely suitable for younger more heads due to wear. Its is also easily scratched and it’s notch- active patients but should be used in the older, lower demand sensitivity may reduce fatigue life of the implant [18, 24]. patients and patients with soft bone, for example rheumatoids as well as in those with acetabular protrusio 10.2.2. Surface Engineering and Modification. Failure rates of 10-23% at 10 years have been reported Establishment and maintenance of a durable connection [37, 55, 58]. The quality of the cementing technique is very between the implant and host skeleton underpins the success important in acetabular cup loosening. Causes of failure of cementless fixation. Success is ensured by close contact include poor operative technique [61, 62], failure to remove between the implant surface and host bone, minimizing all articular cartilage at the periphery or poor pressurization relative motion at the interface and appropriate surface. In our practice we routinely use flanged sockets to characteristics. Surgical trauma during preparation of establish the best bone-cement interface at the time of the bony bed for the implant and placing the implant into the surgery. Indeed, these have been shown to be effective with canal is thought to stimulate mesenchymal stem cells on the higher peak pressures and higher intruded cement volumes endosteal surface to become osteoblasts, subsequently start being obtained [62-64]. Poor acetabular bone quality on laying down extracellular matrix which eventually becomes average reduces the longevity of the cup and therefore the mineralized via the intramembranous pathway. Implant acetabulum must be reconstructed with grafts or metallic stability at the time of surgery and intimate contact between devices when necessary before cementing in the cup, to the implant surface and viable host bone are critical to ensure sufficient fixation. success. It is important to keep the distance between the Despite problems with cemented implants, long-term implant and host bone to a minimum of 50μm to enable results do demonstrate that cement fixation does provide osteogenic cells the bridge the bone-implant interface. stable long-term fixation. Attention to detail of the technical Although interface gaps of up to 1mm can be bridged, there aspects of cement fixation and a good understanding of the is a risk of excessive interface motion. Excessive interface basic science principles underpinning this method are motion of more than 30-150μm leads to fibrous tissue paramount for success and we continue to use it extensively formation rather than a rigid bony interface [19, 68]. in our practice. Achieving rigid fixation with the line-to-line reaming 174 The Open Orthopaedics Journal, 2010, Volume 4 Mirza et al. technique may sometimes be challenging and the use of causing stress shielding and proximal bone loss. In addition, supplementary fixation with screws may be required to they may cause increased thigh pain. One FEA study maintain rigid fixation. Screws however, may increase the demonstrated that coating should be present on the upper effective joint space, decrease the acetabular surface area for 50% of the implant to avoid stress shielding. It has been bony in-growth and potentially increase the extent of shown that proximal bone loss from stress-unloading of the osteolysis. In addition, screw backout onto the backside of proximal femur using extensively porous coated implants is the PE insert has been demonstrated to cause backside wear progressive, albeit stabilizing after two years. However,. Our preference is not to use screws if a reasonable fix in a revision situation, extensive porous coating is beneficial is achieved as studies have demonstrated a 99% 12 year as distal fixation is to be relied upon for implant stability survival of acetabular shells without screw fixation. and prostheses which have limited proximal coating Recently, better designed locking mechanisms between the have significant failure rates in this situation as this relies on shell and the liner have replaced previous designs which maximum bone contact in the metaphysic where bone stock allowed motion between the PE liner and the shell, is often deficient. contributing to backside wear. Press-fit systems where Survival rates as high as 98.8% at 10-year follow-up the bone is reamed to 1-2mm smaller than the actual have been demonstrated for uncemented coated acetabular component, depend on hoop stresses to achieve primary cups and we continue to use them in our higher demand stability and do not require supplemental fixation. However, younger patients. Similarly, for uncemented femoral one needs to be careful about the risk of fracture. In our components, loosening rates of now less than 0.5% annually experience, under-reaming by 3-4 mm significantly increases can be achieved. Intraoperative fractures are more the risk of intraoperative fractures. Detection of an common in femoral uncemented components and one intraoperative fracture is a good indication for acetabular must be careful especially in the calcar region when inserting screws. Trabecular metal is a new type of material made them. of tantalum used for porous monobloc acetabular implants with an integrated PE liner, potentially eliminating the risk 10.3. Bearing Surfaces of backside wear [72, 73] and has shown promising bone Hard-on-soft bearings have included metal on PE ingrowth and mechanical fixation and, in an experimental articulations and more recently ceramic on PE. The metal on model, superior bony gap healing and less migration of PE PE bearing has been the most common bearing surface and particles in peri-implant tissue. However, long-term has proved to be economical due to it’s low cost and ease of results are lacking. manufacture combined with good long term results Studies have demonstrated that in porous coated especially in lower demand individuals. Titanium heads have implants, pore sizes that optimize bony ingrowth range been least favourable due to high rates of volumetric wear, between 100-400μm [75-77] and optimum pore density is notch sensitivity and third body wear [20, 85] and is no 40-50% , beyond which the porous coating may actually longer used for head manufacture. Cobalt-chrome on PE be sheared off the implant. It is thought that in grit-blasted bearings have had reasonable long-term success. The prosthesis, a large proportion of the surface needs to be problems of degradation of mechanical properties of PE and covered, and the higher the surface roughness, the higher the increased wear rates [86, 87] due to previous gamma risk of abrading cortical bone, producing metal debris. irradiation in air and oxidation on the shelf [70, 88, 89] have The extent of porous coating in primary hip arthroplasty now partially been overcome by enhanced cross linking and is controversial. We generally use implants that have been stabilization procedures that include exposing it to a circumferentially porous coated to encourage bony ingrowth sequence of radiation in an inert environment combined with from all directions and to avoid development of stress risors an annealing or melting procedure. PE now has at points between coated and uncoated sections in non- excellent wear properties due to processing to achieve circumferentially coated stems. Non-circumferentially coated enhanced crosslinks producing ultra-high molecular weight stems fail to establish bony ingrowth around the entire PE (UHMWPE). This has led to improved surface hardness perimeter of the proximal femur and allow access of PE and enhanced resistance to adhesive and abrasive wear, debris to the femoral diaphysis. In effect, the effective joint albeit at the expense of increased brittleness and space is increased causing femoral diaphyseal osteolysis susceptibility to fracture. The use of vitamin E and other. Using circumferentially coated stems allow bony additives can potentially improve mechanical properties of ingrowth around the entire circumference of the stem, PE. Despite this, metal-on-PE bearings still have the highest thereby sealing off the effective joint space from the femoral wear rates when compared to other combinations at diaphysis. Indeed, circumferentially coated stems have 0.28mm/year of volumetric wear. Using ceramic heads performed better than their non-circumferentially coated which have better scratch resistance than metal heads in counterparts with regards to osteolysis [78, 79]. In our these bearings have led to more favourable wear rates of less practice we tend to use femoral stems that are proximally than 150 μm per year. This constitutes a 50% decrease in circumferentially coated for our primary THAs. Proximally wear rates reported for traditional metal-on-PE bearings [91- coated femoral stems achieve fixation in the proximal 93]. metaphyseal region, thereby transmitting forces in a more Hard-on-hard bearings currently include metal-on-metal physiological fashion to avoid stress shielding. Fully coated and ceramic-on-ceramic. Metal on metal articulations are implants achieve fixation throughout the length of the made of cobalt-chrome, providing high ultimate strength, implant due to the large surface area of coating. Substantial superior biocompatibility, excellent corrosion resistance and amounts of force loads are thus transmitted to the more distal reduced tendency to fretting. After an initial ‘wearing in’ aspects of the femur at the expense of the proximal parts, Basic Science Considerations in Primary Total Hip Replacement Arthroplasty The Open Orthopaedics Journal, 2010, Volume 4 175 phase, they have a self-polishing property and produce heads without producing prohibitive amounts of wear debris. markedly reduced volumetric wear estimated at 2.5- To avoid early impingement, it seems that the ratio between 5.0μm/year [91-93]. Furthermore, particles produced are of the femoral head and neck diameter should be over 2:1 smaller size and number [94,95] which are thought to be less. biologically active. However, increased levels of metal ions in blood, lymphatic tissues, urine and other tissues have been demonstrated with long term effects not yet elucidated. Potential carcinogenic effects of metal ions are a concern O although no cases of neoplasia have been correlated 100 directly with these articulations. There may be a potential for 22 development of delayed-type metal hypersensitivities with MOM articulations. Aseptic lymphocytic vasculitis associated lesions (ALVAL) are characterized by the presence of extensive perivascular or diffuse infiltrates of both B and T lymphocytes [98-101]. large areas of necrosis may be caused by these reactions in spite of there being minimal wear debris within these areas. Ceramic-on-ceramic articulating surfaces combine properties of a high strength, scratch resistant material with very low coefficients of friction averaging 0.02, mimicking those of a normal joint. Their superior wettability and O hydrophilic surfaces aid in lubrication when compared to an 28 120 MOM articulation of the same diameter. This increases with head size, with the potential ‘holy grail’ of hydrodynamic lubrication in a hard-on-hard articulation. COC articulations reduce abrasive and adhesive wear as well as the number of wear particles produced that may take part in the loosening process or third-body wear. Studies on cadaveric alumina heads showed hardly any change in surface Fig. (4). Schematic diagram showing the effect of head size on roughness or roundness of the implant and scanning electron primary arc range. microscopy studies have shown hardly any deformation in With hard-on-hard articulations, the larger the head size surface compared to unused ceramic heads [103, 104]. the better due to the ever-increasing role of hydrodynamic Currently, COC articulations have the best wear profile, with lubrication and low volumetric wear. In our experience, this annual wear rates of 0.5-2.5μm/year and volumetric wear has a significant effect on stability not just due to improved averaging 0.004mm/year. The particles produced are head-neck ratios but also also due to the effect of suction on small and do not activate the osteolytic pathway in the same containment and an increased ‘jump gap’. manner as PE does. The current reported failure rates of ceramic heads are less than 0.004%. However, ceramics Constrained acetabular liners provide excellent stability are expensive to produce, are unstable for certain geometric but at the expense of a reduced primary arc range. In shapes and are brittle and if they do fail, they may fail addition, large forces may be transmitted to the acetabular- catastrophically. bone interface, causing mechanical loosening. In our practice we occasionally use these sort of components for neck of 10.4. Prosthetic Head and Head-Neck Ratios femur fractures in the elderly, lower demand patients and The prosthetic head plays an important part in generation patients with other conditions that may predispose to of wear particles, a factor implicated in the loosening dislocation, for example neurological conditions. process. A smaller dimension head causes less volumetric 10.5. Stem Shape wear due to having a smaller arc of motion than a larger head. The distance between two points that the head has to Cemented components are necessarily of lesser diameter travel is also smaller for the same angle compared to a larger than uncemented ones to allow room for the cement. In head (Fig. 4). A correspondingly larger head has to travel a uncemented components, the stem diameters are greater in greater distance for the same arc of motion and hence order to achieve maximum bone-implant contact, to potentially generates more wear particles. However, linear encourage bony ingrowth. There are a variety of stem shape wear in smaller heads is greater because the JRF is designs in use. Circular or elliptical sections have the least distributed over a smaller area. The head size is of particular potential for bony attachment, except when a good initial fit relevance in hard-on-soft articulations such as metal-on-PE. is obtained. Corners that cut into bone have been Maximizing the femoral head-to-neck ratio using larger successful in reducing torsion when combined with certain heads and smaller diameter necks improves head-neck ratios surfaces. These achieve rotational stability as do the ones and excursion distances. In our experience, when considering with longitudinal cutting flutes. The larger uncemented a hard-on-soft articulation, a 28mm head probably represents stems rely on a wedge fit of the prosthesis into the proximal a trade-off between volumetric and linear wear and femur, including multiple contact points of the stem on also produces a reasonable range of motion, although cortical bone, the aim being to achieve an optimal fit both advances in PE hardness have allowed the use of 32mm proximally and distally, to achieve axial and rotational 176 The Open Orthopaedics Journal, 2010, Volume 4 Mirza et al. stability. In general, shorter stems are used for primary. Increasing the lateral offset increases the size of the THRs whereas for revisions we may use longer ones. This is abductor lever arm and abductor tension and reduces the because of the large cavitary defects often present in the JRF. However, increased torsional stresses on the stem and a proximal femur in a revision setting, which require potential for early loosening as well as trochanteric bursitis bypassing due to their poor capacity to take load. are potential problems with this. The vertical height or neck Biomechanical studies have shown that the stress pattern of length also plays a part in appropriate abductor tensioning tubular bone returns to normal at a distance of two bone and also controls leg length. diameters from the most distal defect. Therefore, the In general, following THR, an average femoral offset of length of the stem must bypass the most distal bone defect 45mm produces physiological loading. However, in by at least 2-3 internal bone diameters to ensure stability. In our practice we routinely use templating to determine the addition, at least 4-5cm of intimate contact between the optimum height and offset for the individual patient. Lack of femoral isthmus and the implant is necessary if distal restoration of femoral offset leads to abductor weakness and fix is being relied upon for initial stability in a revision limping. A modular prosthetic system provides a situation. simple way of adjusting neck lengths and offset in patients Dealing with cavitary defects in revision surgery is undergoing THA. A system with variable neck lengths or challenging and in our experience, impaction bone grafting modular heads with variable internal recesses make simple using morselized allograft has proved to be a useful adjustments to neck lengths and enable achievement of technique. Impaction bone grafting has the advantage of correct leg lengths. Some systems provide different stem potentially reconstituting bone stock and allowing the offset sizes and one can also adjust these parameters to a surgeon to fashion the graft to the defect at the time of certain extent by the depth of prosthetic insertion. We use surgery and can be used with the addition of cortical the lateral decubitus position for our THRs and it has been support in form of meshes, strut grafts and plates depending postulated that leg length discrepancies are more likely to on the specific scenario presented. occur in this position. Therefore, in addition to preoperative templating, we use various techniques recommended Designs with a lateral flare attempt to maximize proximal to prevent such discrepancies including assessing the fit and fill such that the length of the stem may be reduced patient’s feet in symmetrical knee flexion, measuring the. In contrast to cemented stems that have smaller cross height of the femoral cut from the top of the lesser trochanter sectional areas, in uncemented stems the prosthesis is meant to fill the canal and hence must be of sizeable diameter and and performing the osteotomy at the level determined by the preoperative template, assessing the relationship of the feet ideally achieve a greater than 90% fill. Bending with the knees bent equally after trial reduction and the stiffness however, is proportional to the fourth power of the relationship of the greater trochanter with respect to the prosthetic diameter. Therefore, larger diameter stems may femoral head centre before and after femoral neck cause more stress shielding than smaller diameter ones [111, osteotomy. 112]. However, if a good proximal fit and fill is obtained and the stem is tightly wedged in the femur, high circumferential 10.7. Component Alignment tensile stresses as well as up to 50% of the compressive stress component is produced , hence reducing bone It is generally accepted that orientation of either loss. Using a material with a lower elastic modulus may component to an excessive degree may predispose to permit the use of larger stems without the penalty of dislocation irrespective of the surgical approach used increased rigidity. Stem stiffness is an important to implant the prosthesis. Many prosthetic systems generally design variable that determines bone remodelling and replicate the normal 10-15 degree femoral anteversion and using a less stiff material has been shown to reduce stress studies have demonstrated a stem side anteversion angle of shielding in the proximal part from 26-75% in the canine upto 15 degrees to be optimal [122, 123]. Optimum model. positioning of the acetabular cup is more controversial and does affect stability quite markedly. The surgical approach to 10.6. Prosthetic Height and Offset (Abductor Tension) the hip may affect the degree of anteversion used and surgeons may use a greater degree of anteversion using a Many of the prosthesis in current use have a modular head attached to the neck by a taper configuration. Cyclical posterior approach to prevent dislocation. For posteriorly implanted hips, 15-20 degrees of anteversion has been shear stresses across the interface between the two parts shown to result in excellent stability [124, 125]. The vertical intended to be statically fixed together may lead to fretting orientation of the acetabular cup also affects stability and corrosion which was a concern using stainless steel those inserted with a large coronal tilt angle are at increased particularly in combination with ceramic heads. However, risk of superior dislocation and generally, a 30-50 degree with the use of newer metal alloys, this problem is now less common. range for this angle is acceptable. In our practice we often use the transverse acetabular ligament (TAL) as a guide to One of the primary goals of THA is to position the acetabular orientation in our primary THAs as this is a fairly primary arc range of the prosthetic hip in the centre of the constant landmark in most native hips. The degree of functional range of motion required by the patient, in order acetabular coverage is also important for stability and long- to optimize the range of motion and reduce the chances of term success. If less than 70% of the trial component is in dislocation. The position of the prosthesis in terms of contact with bone, we use a cortico-cancellous fragment cut neck length and lateral offset is critical and must provide from the femoral head and fixed into the cranial wall of the adequate resting abductor tension for joint stability and acetabulum with two screws to augment the superior wall. soft-tissue balancing is of increasing importance in THR The use of posteriorly elevated acetabular liners may help Basic Science Considerations in Primary Total Hip Replacement Arthroplasty The Open Orthopaedics Journal, 2010, Volume 4 177 prevent posterior dislocation in certain situations, albeit at 10.10. The Future of Total Hip Arthroplasty the risk of posterior impingement. In some cases it may not THRs are being performed in an increasingly younger be possible to control the anteversion of the stem that and more active patient age group compared to earlier years. accurately, especially in cementless stems, and in this case The main cause of failure in these patients remains loosening the cup anteversion can be adjusted to provide a mean combined anteversion of 35 degrees with a safe zone of 25- due to osteolysis and the focus in future is going to be on extending the durability and survivorship of these 50 degrees. components in a younger patient age group. 10.8. Modular Designs There may be future interest and developments in the The use of modular prostheses has partially overcome the pharmacological inhibition of the osteolytic response as well highly unpredictable anatomy often encountered in revision as the developement of novel materials or surfaces that may situations. A modular prosthetic system provides a simple enhance bony in growth onto implants. way of adjusting the vertical height and offset. Variable neck There is currently on going controversy regarding the use lengths or modular heads with variable internal recesses of minimally invasive surgery in THR, with it’s proponents make simple adjustments to neck lengths. Some systems saying it causes less tissue trauma and blood loss and less enable independent sizing of proximal and distal parts of the instability and faster recovery. However, others have stem, and in some, the materials of the proximal and distal reported a greater degree of tissue damage, femoral fracture parts of the stem can be varied to reduce stress shielding. and that nerve damage and component malpositioning is However, having sections of the stem with different elastic greater, in addition to having a steep learning curve with no moduli may lead to stress risors in the stem, with a potential significant differences in outcomes three months after the for failure. operation compared to conventional THA [141-143]. This Soft-tissue balancing is of increasing importance in THA controversy should be resolved in the next few years as more and there have been sex differences demonstrated between data becomes available on outcomes as well as experience women and men, with women having shorter femoral necks, with the techniques increase. thinner femoral shafts, lower cervico-diaphyseal angles, Optimum positioning of the femoral and acetabular lower femoral offsets and greater femoral neck anteversion components have led to various navigation systems being [5, 127-130]. Modular stems may have some theoretical developed whose role in day to day THR is yet to be advantages over monobloc stems in that they allow determined. At present, navigation techniques are not adjustment of the cervico-diaphyseal angle, lateral offset, recommended as a routine procedure. However, these neck anteversion, neck length and lower limb length systems have been shown to be useful in some studies, independent of stem size and length. In our practice, particularly with positioning of the acetabular component we use these components in complex situations with and the use of navigation may become more variation in different anatomical combinations, for example widespread in conjunction with minimally invasive in high grade DDH (thin shaft, long neck, high offset), large techniques for THA. shaft/short neck/high offset, and in situations where there are other mismatches between stem size and neck length or ABBREVIATIONS offset and where there is an excessive amount of anteversion. THA = Total Hip Arthroplasty These components have however been associated with a failure rate of about 0.027%. THR = Total hip replacement 10.9. Custom Designs NHS = National Health Service Custom designs are mainly used in revision situations OA = Osteoarthritis because of the highly distorted and variable anatomy of the TNF = Tumour Necrosis Factor femoral canal and acetabulum due to to large cavitary defects in these situations. Three-dimensional geometry of the HA = Hydroxyapatite femoral canal and acetabulum is usually determined either FEA = Finite Element Analysis by serial radiographs or CT reconstruction [133, 134], PMMA = Polymethylmethacrylate or direct shape determination at surgery [135, 136] and these implants have shown some success in terms of stability and PE = Polyethylene bone preservation in revision situations. 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