LOCEq 1-2 Forelimb Handout 24-25 PDF

Loc 1&2 - Introduction to equine locomotion / Equine forelimb Loc 1&2 - Introduction to equine locomotion / Equine forelimb Learning Outcomes: Learning Outcomes (contd.): Define the term “cursorial specialisa...

Loc 1&2 - Introduction to equine locomotion / Equine forelimb Loc 1&2 - Introduction to equine locomotion / Equine forelimb Learning Outcomes: Learning Outcomes (contd.): Define the term “cursorial specialisation” and understand that adaptation of Give a brief description of the form and function of the joints of the equine the equine limb for speed has involved sacrificing versatility. thoracic limb, their synovial compartments and notable ligaments. List the features of equine limbs that demonstrate cursorial specialisation. Describe the suspensory apparatus and explain its functional significance in Discuss how some of these anatomical adaptations, whilst promoting fast supporting the posture of the equine distal limb. running, have left the equine limb vulnerable to injury. Describe the arrangement of the tendons of the equine distal limb and explain Discuss the general form and function of the equine thoracic limb, and the why they are vulnerable to injury. relevance of limb conformation. Recall the extrinsic and intrinsic musculature of the thoracic limb with Describe the skeleton of the equine thoracic limb, focussing on features emphasis on features specific to / notable in the horse. particularly relevant in the horse and landmarks palpable in the live animal. Describe the passive stay apparatus of the forelimb and explain the relevance List the different types of synovial structure found in the equine limb, briefly of the arrangement to the equine lifestyle. explaining the function of each and their clinical relevance. Recognise the major synovial structures of the equine forelimb. CLINICIANS REQUIRE A SOUND UNDERSTANDING OF Cursorial specialisation: THE FUNCTIONAL ANATOMY OF THE EQUINE Anatomically adapted for fast running LOCOMOTOR SYSTEM The equine limb shows extreme specialisation for running. Most horses are bred and kept for purposes that make demands on their limbs Cursorial specialisation generally involves limb adaptations that help to:  LAMENESS generates a large proportion of equine veterinary work Elongate limb, increasing stride length and range of motion A good understanding of functional anatomy is key Decrease distal limb mass (and therefore inertia) Diagnostics / examination more complex than for small animals Note features common to other cursorial quadrupeds, esp. proximal limb e.g. e.g. manipulating joints / assessing reactions of a large animal - Shoulder girdle as synsarcosis (no clavicle) – increased ROM of scapula N.B. Owners may be knowledgeable! - Large sacropelvic angle, nonrigid sacroiliac joint – pelvic girdle rotations Economics - companion animal or valuable commodity – or both? contributes to the HL stride length Horses are HEAVY animals and cannot manage with non-functional limbs Other features are particularly well-developed in the horse – e.g. posture, loss / elongation of skeletal elements, proximal concentration of limb mass. Attain HIGH SPEEDS  Large forces on single digit Overall, greatly increased efficiency of movement in sagittal plane  Potential for catastrophic failure - Contributes to stride length and stability when moving at speed - At expense of versatility (esp. horse) - rotation, pro/supination, ab/adduction Raising manus / pes from plantigrade posture effectively elongates the limb EQUINE LIMBS – CURSORIAL SPECIALISATION UNGULIGRADE - weight borne on the tip of the digit, protected by hoof Distal limb bones elongated++ (compare digitigrade – e.g. dog / cat; plantigrade – e.g. bear / human) Greater proportion of overall length e.g. metapodials / phalanges Reduction / loss of skeletal elements e.g. MC/MT II and IV much reduced, fused with III and nonweightbearing Fusion of bones of antebrachium / crus (sagittal movement only) Single digit Muscle mass concentrated proximally Reduces mechanical advantage - BUT strength is not the priority: a) small changes in muscle length  large range of distal limb motion b) the speed of movement of distal end of limb is increased BUT note implications for injury and healing Long tendons connect limb muscles with the distal skeleton Reduce distal mass, but also highly specialised in the horse Key role in overall locomotor efficiency Storage/release of elastic energy Shorter ME (moment arm of The forelimb supports ~ 60% bodyweight muscle effort) means lower Incidence of lameness FL > HL (3:1?) mechanical advantage – but movement generated around each joint is additive – Shoulder and elbow joints are moderately angled – from elbow to optimises metacarpophalangeal joint the skeleton forms a bony column. a) range of motion b) speed of movement There is considerable variation in limb shape / form with breed and type of the limb more distally However, there are certain requirements for good function and movement – “conformation” Limb should appear straight when viewed from the front. By contrast, the more upright A straight line dropped from the point of the shoulder should bisect the limb and hoof. Adapted from Biewener, JEB 2005 Muscle Effort posture (compared with e.g. A straight line from the tuber spinae should bisect the limb craniocaudally to the level smaller, crouched mammals) of the fetlock, passing just caudal to the hoof. reduces the moment arm of the ground reaction force (GRF) during limb loading (MG, left). Mediolateral or craniocaudal deviations can: GRF As GRF is what bone / muscle / - lead to increased stresses / strains ME tendon must withstand during MG - predispose to injury weight bearing, this helps to offset the influence of increased size / “Base narrow”, “base wide” etc. – alters the weightbearing axis of the limb – Crouched Upright body mass. e.g. base narrow – more weight borne on lateral aspect; base wide – more towards medial.  SKELETON SCAPULA – Palpable tuber spinae Marks level at which horse’s body weight can be thought of as acting down through the limb at rest Note large scapular cartilage at dorsal extent – attachments of muscles of pectoral girdle - forms part of the withers No acromion Suprascapular nerve vulnerable to injury “Sweeney” – atrophy of supra / infraspinatus mm. HUMERUS – ”Point of the shoulder” is the cranial part of the greater tubercle of the humerus Bones of the left carpus - ANTEBRACHIUM – schematic Ulna fused with radius (ends mid-antebrachium) → no capacity for supination or pronation Med Cranial Lat Dors Lateral Palm CARPUS – 7 or 8 bones, distinct proximal and distal rows Accessory Medial Radial Intermediate Ulnar ^ Lateral (1) 2 3 4 First carpal bone C1 is inconsistent Differentiate from a bone chip! Accessory carpal bone Palpable landmark, caudolateral Consider when bandaging – risk of pressure / rubbing N.B. For bones of carpus and tarsus, metapodials and digits, numbers start at the medial side and increase towards lateral.  MCIII METACARPUS (contd) METACARPUS – Suspensory ligament - the common interosseous muscle, All weight borne through MCIII - cannon bone highly specialised and almost completely fibrous in the horse. Lies on the palmar aspect of MCIII, between splint bones. Distal condyle has a median sagittal ridge Crucial role in supporting limb posture – see later. Ridge sits in groove at proximal aspect of P1 – predisposes to sagittal fracture of P1 (“screwdriver fracture” / “split pastern”) The splint bones (MCII & IV) are joined to MCIII by ligamentous connections (ossify with age) P1 Ligament / periosteum may become inflamed New bone formation leads to bony swelling – “splints” MCII & MCIV / Medial & lateral splint bones Common! Esp. poor conformation, excessive training Much reduced and non-weightbearing Terminate mid-distal MCIII, with palpable “button” at PROXIMAL SESAMOID BONES - distal end Joined by and embedded in dense connective tissue / fibrocartilage – intersesamoidean ligament – so function as a single unit during locomotion N.B. Terminology – metacarpus (a region of the limb) vs. metacarpal (a bone) i.e. each forelimb has one metacarpus, but three metacarpals – MCII, MCIII and MCIV. DIGIT (III): SYNOVIAL STRUCTURES OCCUR THROUGHOUT THE EQUINE LIMB IN THE FORM OF BURSAE, TENDON SHEATHS & JOINT CAPSULES PROXIMAL PHALANX P1 – “Long pastern” Bursae - Offer protection against pressure Subtendinous / subligamentous - e.g. bicipital MIDDLE PHALANX P2 – “Short pastern” Subcutaneous - often adventitious - e.g. “capped elbow” Tendon sheaths - lubrication to reduce friction DISTAL PHALANX P3 – “Pedal bone” (or “coffin bone”) Suspended within hoof capsule – laminar junction (see L3-4) Particularly important are those of the digital flexor tendons: Collateral cartilages extend wings on palmar aspect **Carpal sheath (FL), tarsal sheath (HL) and digital sheath** Injury here presents complex problems and DISTAL SESAMOID (NAVICULAR) BONE – Joint capsules - may have capacious recesses - “pouches” Boat-shaped, bears DDFT over DIP joint Increases angulation of tendon over joint NB – For metapodials and digits – convention is to use Roman numerals working medial-lateral For phalanges – use Arabic numerals working proximal-distal.  IT IS IMPORTANT TO BE AWARE OF THE LOCATION AND EXTENT OF JOINTS OF THE FORELIMB – SYNOVIAL STRUCTURES All are restricted to flexion / extension i.e. sagittal (or near-sagittal) movement - Swelling / increased fluid (effusion) may represent different processes configuration of joint surfaces, collateral ligaments, surrounding muscle etc. – e.g. inflammation, infection (especially due to penetration / a wound) Synoviocentesis – the act of puncturing a synovial cavity with a hollow needle SHOULDER – May be performed for different reasons: Muscle masses supporting laterally & medially → very STABLE joint - Diagnostic (fluid sampling; anaesthetising joint for lameness exam) Tendons of infraspinatus (lateral) and subscapularis (medial) - Therapeutic (e.g. instilling medication into the joint) mm. serve as collateral ligaments for the shoulder joint through their insertions on the proximal humerus. Requires a good understanding of the anatomy! Scapulohumeral joint is a large synovial space, but note Many of the cavities are more extensive than might be expected… also several synovial bursae in the region (protecting tendons as they cross bony surfaces) e.g. Digital flexor tendon sheaths E.g. bicipitial bursa - tendon of biceps brachii m. Carpal sheath – extends about 1/3 of the way down metacarpus Digital sheath – extends from the distal 1/3 metacarpus to mid-P2 ELBOW – Be suspicious of ANY WOUND NEAR A SYNOVIAL STRUCTURE Olecranon is prominent - the major part of the otherwise reduced ulna. Includes foot penetrations – consider the navicular bursa (and DIP joint) Radius provides majority of the weightbearing surface for humeral condyle. The radius and ulna are fused before adulthood – no movement. CARPUS – Compound joint – 2 rows of small carpal bones, numerous ligaments There are three potential levels of flexion: 1) Antebrachiocarpal joint (“radiocarpal”) – most mobile 2) Middle carpal joint – smaller range of motion 3) Carpometacarpal joint – no movement In addition to the fibrous capsule surrounding the whole carpus, each of these is associated with its own discrete synovial joint space / compartment **Synovial spaces of middle carpal & carpometacarpal joints always communicate** (i.e. the two distal joints – contrast the tarsus – see L5-6) Thus anything that enters 2) will be able to enter 3), and vice versa… Local anaesthetic; medication; contamination / infection…. Antebrachiocarpal joint – no communication Note close packing of carpal bones dorsally Contributes to stability at rest – inhibits overextension (see passive stay apparatus) Driven together at fast gaits – leading to risk of fracture – chips, slab # LIGAMENTS OF THE CARPUS – ECR METACARPOPHALANGEAL (FETLOCK, M.C.P.) JOINT – CDE ECOb Numerous attachments connect individual Multiple elements contribute to the articular surface: bones (plus collateral ligaments etc.) LatDE Distal condyle of MCIII Proximal P1 Some of clinical note: Proximal sesamoid bones e.g. accessoriometacarpal ligament DDF Intersesamoidean ligament Connects accessory carpal bone to metacarpus UL SDF A palpable landmark for some procedures Large range of movement (>> P.I.P. and D.I.P. joints) FCU Extensive synovial joint capsule: Dense connective tissue joins bones at their palmar aspect: Synovial membrane extends dorsal and palmar pouches Together, form Palmar carpal ligament the carpal (Distension – “windpuffs” / “articular windgalls”) Smoothes palmar aspect of skeleton (esp. for tendons) canal: Origin of the accessory ligament of the DDFT Conveys digital Note the position of the joint at rest: flexor tendons Flexor retinaculum (within their Extended beyond the vertical - bodyweight of the horse acting down through carpal sheath), the limb would therefore tend to extend it further i.e. over/hyperextension Thickened fascia, binds flexor tendons to the vessels & nerves Supported by suspensory apparatus - crucial for normal posture & function palmar aspect of the limb. to distal limb. (also digital flexor tendons & accessory ligaments) PROXIMAL INTERPHALANGEAL (PASTERN, P.I.P.) JOINT - Equine digit, parasagittal Digital sheath Articulation P1 - P2 section Fetlock joint Synovial joint capsule with dorsal and palmar pouches capsule - palmar pouch DISTAL INTERPHALANGEAL (COFFIN, D.I.P.) JOINT - Common digital extensor tendon Proximal sesamoid P2, P3 and navicular (distal sesamoid) bone - largely enclosed within hoof capsule bone Range of motion is similar to P.I.P. joint (i.e. both