J . small Anim. Pmct. (1975) 16, 21-25.

Lag screw fixation of a patellar fracture C . W. B E T T S *

AND

MICHAEL WALKERt

Department of Small Animal Medicine and Surgery,* Department of Anatomy and Radiology, t College of Veterinary Medicine, University of Georgia, Athens, Georgia 30602

ABSTRACT A case report of a longitudinal patellar fracture that was successfully treated by lag screw fixation is presented. INTRODUCTION There is little information in the veterinary literature pertaining to fracture of the canine patella. An incidence of 1 :1,000 relative to all fractures seen in the dog has been reported (Leonard, 1971), with patellar fractures reported as rare due to the small compact nature of the bone (Hickman, 1964). ANATOMICAL FUNCTION The patella is the largest sesamoid bone in the body. The proximal portion is the base while the slightly more pointed distal portion is the apex (Miller, 1964; Bostrom, 1972). I n the dog, the descending genicular artery is the main blood supply to the stifle joint and sends branches to the medial side of the femoropatellar joint capsule. These branches enter the patella medially through several nutrient foramina (Miller, 1964). The patella is an ossification in the tendon of insertion of the quadriceps femoris muscle and serves t o improve the efficiency of the direction of pull of the contraction ofthe quadriceps. I t protects the tendon and provides a larger bearing surface for the tendon to play on the trochlea of the femur (Miller, 1964). I n man, the patella facilitates extension of the knee-joint by the quadriceps muscle which is the basic factor in maintaining erect posture (Anderson, 1971; Bostrom, 1972). I n animals, the patella acts at its greatest mechanical advantage when the position of 21

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the stifle changes from full flexion to full extension, as when a cat springs (Dingwall, 1970). MECHANISM O F FRACTURE Direct violence results in a variety of fractures. It is the most common cause of longitudinal lateral fractures of the patella which are considered rare in man. T h e position of flexion of the knee-joint at the time of trauma influences the type of fracture seen in humans (Bostrom, 1972). Also, in man, indirect violence most often results in a transverse fracture (De Palma, 1954). These factors would apply to the dog also. Dogs usually hold the limb in flexion which maintains separation of the fracture fragments (Alvarenga, 1973). CASE R E P O R T A 1O-year-old female Greyhound was examined a t the University of Georgia College of Veterinary Medicine on 26 September 1973, for a left hind limb lameness of approximately 2 weeks duration. The lameness had an acute onset, and trauma to the limb was suspected by the owner. T h e owner reported increasing lameness with exercise. Physical examination revealed the dog to be in good general health, but exhibiting a weightbearing lameness of the left rear limb when walking. At rest, the dog

FIGSl(a) and (b). Postero-anterior and lateral radiographs ofstifle 2 weeks after injury.

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preferred to hold the limb off the floor. Careful examination of the limb localized the lesion as being in the stifle as shown by pain on flexion and crepitus in the dorsal aspect of the femoropatellar pouch with the limb in extension. The cruciate ligaments and collateral ligaments were intact and the patella was well seated in the trochlear groove. T h e dog was hospitalized for radiographic evaluation. Lateral and posterior-anterior radiographs of the left stifle (Fig. 1) showed a longitudinal sagittal fracture of the patella with the fracture line extending from the dorsolateral base of the patella to the midline of the apex. The fragments were separated 14 mni. A bone chip was evident on the dorsocaudal margin. Joint effusion was present. Surgical stabilization was decided upon to relieve the animal’s discomfort and to encourage early return to function. A lateral parapatellar approach was utilized to minimize compromise of the medial blood supply. T h e fracture was stabilized with a 12 mm small cancellous (navicular) screw (ASIF-Smith, Kline Surgical Specialties, Philadelphia) utilizing the cancellous lag screw principle (Muller, 1970). Compression was assumed to have occurred because of the formation of an everted ridge of fibrous tissue on the dorsum of the patella a t the fracture site as the screw was seated. Maintenance of the fracture gap on the post-operative radiograph was thought to be due to positioning and intervening fibrous tissue (Fig. 2). Radiographs taken 4 weeks after surgery showed maintenance of fragment

FIGS2(a) and (b). Lateral and postero-anterior radiographs of stifle immediately after operation.

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FIGS3(a) and (b). Lateral and postero-anterior radiographs of stifle 6 months after operation.

alignment with slight evidence of healing. Radiographs made 6 months postoperatively (Fig. 3) showed advanced healing with only a faint fracture line being evident. There was some loss of density in the distal end of the patella. The bone screw was still in place and the stiflejoint did not show effusion. Clinical improvement was evident within a few weeks after surgery. Good range of motion was noted, and no crepitus was palpable. One year after surgery, the dog is still sound. DISCUSSION The prognosis after surgery has been said to be good in all cases in the dog (Hickman, 1964). However, only wiring of transverse fractures was described. Longitudinal patellar fractures in man are treated conservatively unless painful, and then are stabilized by cancellous lag screws (Muller, 1970). Conservative therapy in dogs entails prolonged immobilization of the stifle joint which often results in soft tissue problems. Early surgical intervention to obtain interfragmentary compression is advised to facilitate early return to function. ACKNOWLEDGMENTS

Appreciation is expressed to Ed Davidson, DVM of Apex, North Carolina for his assistance in this case. Also to Mrs Pamela Hobby for typing the manuscript.

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REFERENCES ALVARENGA, J. (1973) M o d . Vet. Pract. 54, 43. ANDERSON,L.D. (1971) Campbell’s Operative Orthopedics. Ed. by Crenshaw, A.H., Vol. I., C. V. Mosby Co., St. Louis. BOSTROM, Am. (1972) Acta Orthopaedica Scand. Suppl. 143, 1. DE PALMA, A.F. (1954) Discares o f t h e Knee. J. B. Lippincott Co., Philadelphia. DINGWALL, J.S. & DOWNEY, R.S.(1970) Mod. Vet. Pract., 51, 31. HICKMAN, J. (1964) Veterinary Orthopaedics. Oliver & Boyd, Edinburgh and London. LEONARD, E.P. (1971) Orthopedic Surtery o f t h e Dog and Cat. W. B. Saunders Go., Philadelphia, London, Ontario. MILLER,M.E., CHRISTENSEN, G.C., & EVANS, H.E. (1964) Anatomy ofthe Dog. W. B. Saunders Co., Philadelphia and London. MULLER,M.E., ALLGOWER, M., & WILLENEGGER, H. (1970) Manual of Internal Fixation. SpringerVerlag, New York, Heidelberg, Berlin.

Lag screw fixation of a patellar fracture.

J . small Anim. Pmct. (1975) 16, 21-25. Lag screw fixation of a patellar fracture C . W. B E T T S * AND MICHAEL WALKERt Department of Small Anima...
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