SURGERY AND DENTISTRY


contribute to their formation.4 SCLs are the most com- mon type of OCLLs in horses. Theories regarding sub- chondral cyst formation include dietary, biomechanical, genetic, and physiological factors. The SCLs are divided into 2 categories: developmental (juvenile) or acquired lesions.6,7,9–11 SCLs are most commonly detected at <2years of age but have been reported in older horses.5 Developmental SCLs diagnosed in young horses are of- tendetected onsurveyradiographsprior to the onset of clinical findings. Acquired SCLs occur as a consequence of trauma or overload of the cartilage and subchondral bone.3,6,10 Several hypotheses about pathogenesis have been raised. One hypothesis states that the focal micro- damage to the trabecule exceeds reparative ability, the subchondral bone collapses, and the cartilage barrier cracks, allowing joint fluid to contact the bone, which activates monocytes, leading to cytokine release and osteoclast activation, osteocyte death, and inhibition of osteoblasts.5,12 The pressurized synovial fluid may alter loading conditions of the surrounding bone and induce a mechanoregulated bone adaptation response.13 Another suggests subchondral lesions are mediated


by damage to the microvasculature with ensuing chondronecrosis. The osteochondral unit is composed of articular cartilage, calcified cartilage, and subchon- dral and trabecular bone, working synergistically to support functional loading of the joint.14 Athird hy- pothesis proposes that altered collagen metabolism maybe anunderlyingpredisposingfactor. The biome- chanical insults may eventually result in osteochon- dral fragmentation of the articular surface or at periarticular locations or in the formation of juvenile subchondral bone cysts.1 A more recent theory states that bone strain is reduced within the SCL and that environment could contribute to the poor bone healing response.2 Lamenesses caused by distalmedial femoral condyle


SCLs are variable in severity and often intermittent. Many treatments have been described varying from exercise restriction, synovial and intralesional steroid injections, to surgical treatments including arthrotomy with cyst debridement, arthroscopic cyst enucleation, cancellous bone grafting, cartilage implantswith fibrin glue overlays, intraoperative synovial and intrale- sional injections with steroids, and transcondylar screws placed in lag fashion.2,15–17 Some of these treat- ments have been used sequentially; exercise restric- tion tends to be the first choice, followed by steroid injection before undergoing surgery.5 Previous reports utilizing transcondylar lag screws


are predominantly in Thoroughbreds (TBs) with very few Quarter Horses included. In those studies, 75% of TB patients demonstrated an increase in SCL density and elimination of lameness, indicating that the screw’s compression helps to promote SCL resolu- tion.18 Thoroughbreds often retire from their rac- ing careers at a young age, making long-term follow-up problematic due to horse movement, confi- dentiality, and complexities in finding new owners. The prevalence ofmedial femoral condyle SCLs is 1.7% to 3.6% in Thoroughbreds and 10% to 13.6% in


458 2022 / Vol. 68 / AAEP PROCEEDINGS


Quarter Horses.2 The purpose of this study is to pres- ent follow-up data in QuarterHorses that were treated with a transcondylar lag screw based on initiation of training, return to their activities or training, sound- ness, and in some cases radiographic assessment. Return to initial activities or training is not an ideal measurement but is themost accessible information in long-termfollow-up.


2. Materials and Methods


Horses Medical records from2014 to 2021 were compiled for 58 QuarterHorses. The horseswere between 1 and 7years of age. Each horse had a medial femoral condyle sub- chondral bone cyst that was treated with a transcondy- lar lag screw. Of the 58 horses, 38 were confirmed by the attending clinician to be lame in a rear limb. The remainder were diagnosed on survey radiographs. Follow-up data included phone calls (47 horses), recheck radiographs (10 horses), and lameness exams (5 horses).


Surgical Technique


For the surgical technique, as described by Santschi et al.,2 the horses were anesthetized and positioned in dorsal recumbency with the affected limb in exten- sion. Potassium penicillin (22,000IU/kg, intravenous [IV]), gentamicin (6.6mg/kg, IV), and phenylbutazone (4.4mg/kg, IV) were administered preoperatively. A 10-MHz linear ultrasound probe was used to examine and identify the medial collateral ligament, medial meniscus, and proximal margin of the articular pro- cess. It is important that the stiflewas not lockedor in tension. Skin staples were placed cranial to the medial collateral ligament 30mm proximal to the proximalmargin of the articular process. A fourth sta- ple was placed 10mm cranial to the medial collateral ligament (MCL) and 15mm proximal to cartilage. In some cases, a lateromedial radiograph was used to reassure the position of the staple in relation to the craniocaudal position of the SCL.2 A 15-mm vertical stab incisionwasmade perpendicular to the skin surface and soft tissues 5mm cranial to the MCL and centered 10mm proximal to the palpable proximal margin of the articular cartilage of themedial condyle.2 Atissue guide and a 4.5-mm drill bit were placed proximal to the medial femoral condyle (MFC) cartilage, angled proximo- distal to intersect the center of the medial intercondylar eminence and drilled until the cyst was entered. The drill insert was then placed into the 4.5-mm glide hole and a 3.2-mmbit was used to continue the tract axially. For assessing the position of the drill bits and screw, intraoperativemultiple caudal to cranial 20° stifleradio- graphs were taken. The 3.2-mm bit was then cleaned of bone before minimally penetrating the axial aspect of the condyle. In some cases, biologic therapy was injected into the SCL before the screw was placed. The hole was measured and tapped, and a properlymeasured 4.5-mm cortical screw was placed and tightened. Most screws were placed across the condyle perpendicular to the


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