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Keywords:

  • horse;
  • stifle;
  • subchondral cystic lesion;
  • arthroscopy;
  • meniscus

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Manufacturer's address
  8. References

Reasons for performing study: Medial meniscal injuries and subchondral cystic lesions (SCL) are known to occur independently within the medial femorotibial (MFT) joint in horses. However, there are no reports of a potential clinical relationship between these 2 types of lesions.

Objectives: To: 1) document the concurrent presence or sequential development of medial meniscal and SCL of the medial femoral condyle within the MFT joint; and 2) determine the prognosis with both types of lesions.

Methods: Retrospective case series of horses with both a medial meniscal and SCL of the medial femoral condyle identified concurrently or sequentially by radiography, arthroscopy or post mortem examination. Case records and radiographs were reviewed, and a telephone survey of referring veterinarians, owners and trainers was conducted.

Results: Twenty-one horses (9.1% of all horses undergoing MFT joint arthroscopy) were identified to have both a medial meniscal injury and SCL of the medial femoral condyle. Thirteen horses had both abnormalities identified concurrently, 6 developed a meniscal lesion subsequent to SCL debridement, and 2 developed a SCL subsequent to a medial meniscal injury. Only 4/19 horses were classified as successful and returned to their intended use. The severity of the meniscal injury was significantly associated with the severity of lameness but not with outcome.

Conclusions: A low percentage of horses may develop both a meniscal injury and SCL of the medial femoral condyle within the MFT joint and have a poor prognosis.

Potential relevance: Trauma to the MFT joint may lead to both meniscal and subchondral bone damage of the medial femoral condyle that may be recognised concurrently or sequentially.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Manufacturer's address
  8. References

Subchondral cystic lesions (SCL) in the medial condyle of the femur may form in association with osteochondrosis (McIlwraith 1982, 1993; Scott et al. 2004; Semevolos and Nixon 2007) or as a traumatically induced lesion (Verschooten and deMoor 1982; Jeffcott and Kold 1982a; Jeffcott et al. 1983; Kold and Hickman 1986; Ray et al. 1996). Medial meniscal injuries have been reported to result from trauma to the equine MFT joint (Walmsley et al. 1995). These joint abnormalities usually occur independently in most horses. A previous study on the pathogenesis of traumatically induced SCL demonstrated that articular cartilage injury alone would not induce a subchondral cystic lesion; however, injury that included subchondral bone damage resulted in the development of an SCL (Ray et al. 1996).

Meniscal injuries have been recognised more recently and are thought to account for approximately for 5–12% of all stifle lameness (Jeffcott and Kold 1982b; Walmsley et al. 1995). One study also described that 61 of 80 horses with meniscal injuries had evidence of secondary cartilage damage (Walmsley 2003). Concurrent soft tissue injuries of the cruciate, meniscal and/or collateral ligaments, either alone or combined with meniscal injuries are also known to occur in the femorotibial joints of horses (Trumble et al. 1997; Walmsley 2003). However, the combination of meniscal and subchondral bone injuries has not been reported in horses. The authors have recognised medial meniscal injuries subsequent to SCL debridement and have identified concurrent medial meniscal and SCL of the medial femoral condyle in a small number of cases. A potential cause and effect between meniscal and SCL has not been reported. Therefore, it was the purpose of this study to describe a series of cases in which both medial meniscal injuries and SCL of the medial femoral condyle were identified either concurrently or subsequent to arthroscopic surgery of the MFT joint, and to determine the prognosis for soundness of affected horses.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Manufacturer's address
  8. References

Medical records

Medical records of horses that underwent arthroscopy of the MFT joint from 1996–2006 at the James L. Voss Veterinary Teaching Hospital at Colorado State University or at the Equine Medical Center in Cypress, California were reviewed. Horses were included if they had both a medial meniscal injury and SCL of the medial femoral condyle identified at the time of surgery or developed either lesion subsequent to MFT arthroscopy. Subsequent injuries were confirmed by a combination of radiography, arthroscopy or post mortem examination of the MFT joint. Horses that had other injuries to soft tissue structures (i.e. collateral ligaments or cruciate ligaments) or full thickness cartilage damage other than at the site of the SCL were not included.

Information: This included subject details, use of the horse, duration and severity of the lameness, limb affected and whether or not there was evidence of joint effusion on the initial examination. Lameness severity was graded (Ross and Dyson 2003) on a scale of 0 (no evidence of lameness) to 5 (minimal weightbearing). Joint effusion was evaluated by direct palpation of the MFT or femoropatellar joints, and the results recorded as positive or negative. The presence of an SCL was documented radiographically and radiographic evidence of osteophytes within the MFT joint (proximal-medial aspect of the tibial plateau, proximal intercondylar eminence, or axial aspect of the medial femoral condyle) at the initial evaluation was recorded as either yes or no.

Three different clinical presentations for the combination of medial meniscal and SCL of the MFT joint were identified; 1) an SCL and medial meniscal lesion were identified concurrently at the initial examination and arthroscopy (Fig 1), 2) an SCL was debrided arthroscopically and a medial meniscal lesion subsequently developed (Fig 2), and 3) a medial meniscal lesion was debrided arthroscopically and an SCL subsequently developed (Fig 3). All horses had an SCL in the common axial location on the medial femoral condyle, although the size and characteristics of the lesion were variable.

image

Figure 1. Concurrent SCL and medial meniscal lesion of the MFT joint. This caudal-cranial stifle radiograph of case 12 revealed a large SCL, and the horse had a Grade 3 meniscal lesion diagnosed arthroscopically. The SCL could not be injected or debrided because subchondral bone was covering the opening of the SCL at the articular surface. The lameness in this horse persisted and he was retired to pasture.

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image

Figure 2. Meniscal lesion that developed subsequent to debridement of an SCL of the medial femoral condyle. This post mortem image of case 21 revealed severe damage to the medial meniscus. The horse was initially doing well clinically but became severely lame approximately 4 months after the initial surgery.

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image

Figure 3. An SCL that developed subsequent to debridement of a Grade 3 meniscal lesion. This caudal-cranial stifle radiograph of case 14 was taken 3 months after arthroscopic debridement of the medial meniscal lesion. The SCL of the medial femoral condyle was not present on the initial radiographs. The lameness in this stallion persisted and he was retired to breeding.

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Arthroscopy: Evaluation was performed using the lateral approach to examine the medial condyle of the femur, the medial meniscus, the cranial ligament of the medial meniscus, and both cruciate ligaments (McIlwraith et al. 2005). Information recorded was the presence or absence of a meniscal lesion on initial treatment of horses with a primary SCL or presence or absence of an SCL on initial evaluation of horses with a primary meniscal injury. Treatment of the SCL involved either debridement of the abnormal cartilage and subchondral bone, intralesional injection of corticosteroids into the SCL, or nothing (injection into the SCL could not be performed due to bone present along the articular component). Treatment of medial meniscal lesions consisted of resection of the diseased portion using a motorised tissue resector and/or arthroscopic rongeurs. Treatments of horses were recorded as either SCL or meniscal debridement alone, SCL injection with or without meniscal debridement, or attempted SCL injection (no treatment of SCL) with or without meniscal debridement.

Meniscal lesions were evaluated arthroscopically or at post mortem (in 2 horses subjected to euthanasia) using the grading system established by Walmsley (2003) (Grade 1= a tear of the cranial ligament extending into the meniscus but without separation of the tissues, Grade 2= a complete tear of the cranial pole of the meniscus and the cranial ligament, whose limits were visible arthroscopically and Grade 3= a severe tear of the meniscus and cranial ligament that extended beneath the femoral condyle and whose limits could not be seen). Grades 1 and 2 meniscal lesions (complete lesion visible arthroscopically) were grouped in this study because distinction between these 2 grades could not be determined definitively from the surgery reports. The articular characteristics of the SCL and the focal cartilage abnormalities on the medial femoral condyle seen arthroscopically were also recorded.

After care: Horses received penicillin G potassium (20,000 u/kg bwt i.v.), gentamicin (6.6 mg/kg bwt i.v.), and phenylbutazone (4.4 mg/kg bwt per os) prior to surgery. Another dose of penicillin was given i.v. 6 h after surgery and phenylbutazone (2.2 mg/kg bwt per os b.i.d.) was continued for 5–7 days post operatively. Owners were instructed to stall confine the horses for 4–6 weeks and then turn into a small paddock or exercise run for an additional 4–6 weeks. Hand walking commenced at 2 weeks post operatively starting with 5 min daily and increasing by 5 min each subsequent week up to 30 min/day. Owners were instructed to have the horses re-examined 2–3 months after surgery by the attending surgeon or by their regular veterinarian. Horses were usually permitted access to a small pasture or turn-out from 3–6 months after surgery. Regular exercise was not recommended to be resumed for at least 6 months.

Outcome measures were determined by telephone interviews with the owner, trainer, or referring veterinarian or by re-examination. Minimum time for follow-up was one year. Follow-up information obtained included the soundness of the horse and whether the horse had returned to its previous level of activity. Outcome was considered successful if the horse had returned to its previous intended use with no signs of lameness. If the lameness persisted or the horse could not be used for its intended purpose, the outcome was considered unsuccessful.

Statistics

Proportions between groups were compared for subject details, clinical parameters (lameness grade, lameness duration, limb affected, presence or absence of joint effusion), radiographic abnormalities (presence or absence of osteophytes in the MFT joint), surgical findings (meniscal grade, type of lesions, treatments used for SCL) and outcome (successful or unsuccessful) using Fisher's exact test analysis. The statistical analysis was performed using a software program1. Differences were considered significant when P<0.05.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Manufacturer's address
  8. References

Stifle arthroscopy of the MFT joint was performed on 230 horses at the 2 hospitals during the study period. Twenty-one horses (9.1%) had concurrent medial meniscal and SCL identified at surgery or sequentially developed either lesion at a later time (Table 1). There were 13 Quarter Horses, 2 American Paints, 2 Arabians, 2 Thoroughbreds, one Warmblood and one Appaloosa; 11 geldings, 6 mares and 4 stallions; age range 2–14 years, mean ± s.d. 6.7 ± 4 years, median 7 years.

Table 1. Clinical data of 21 horses with combined medial meniscal injuries and SCL of the medial femoral condyle within the MFT joint
CaseSexAge (years)BreedUseLimbLameness gradeJoint effusionLameness duration (days)Presence of osteophytesType of LesionMeniscal lesion gradeOutcome
  1. ND = no data reported; Sex: G = gelding, F = female, M = stallion; ND = not determined; Breed: QH = Quarter Horse; TB = Thoroughbred; App = Appaloosa; WB = Warmblood; Lameness grade: Scale of 0–5; Type of lesion: Concurrent = both present at time of arthroscopy, SCL then meniscus = primary treatment for SCL and then developed meniscal lesion, Meniscus then SCL = primary treatment for meniscal lesion and then developed an SCL; Meniscal lesion grade: 1/2 = lesion visible with arthroscopy (mild/moderate), 3 = complete lesion not visible with arthroscopy (severe); Outcome: Successful = able to return to performance, Unsuccessful = not able to return to performance, *= used for light riding.

1M7QHCuttingR2Yes30NoConcurrent1/2Unsuccessful
2G13ArabShowR4Yes120YesConcurrent1/2Lost to follow-up
3M2QHCuttingR4No60NoConcurrent3Euthanasia
4G3ArabShowR2Yes700YesSCL then meniscus1/2Successful
5G4QHNDRNDYesNDYesConcurrent1/2Successful
6G11TBJumpingR2YesNDYesSCL then meniscus1/2Successful
7F13QHRacingLNDYesNDYesConcurrent1/2Unsuccessful
8G3AppPleasureR3Yes21NoSCL then meniscus3Euthanasia
9G13TBJumpingR3Yes14NoConcurrent1/2Unsuccessful*
10F14QHNDLNDYesNDYesConcurrent3Lost to follow-up
11M6QHReiningL2Yes210YesConcurrent1/2Unsuccessful
12G7QHCuttingL3Yes360YesConcurrent3Unsuccessful
13G8QHCuttingR2Yes30YesConcurrent1/2Unsuccessful*
14M3QHReiningR3Yes30NoMeniscus then SCL3Unsuccessful*
15F2PaintPleasureR2Yes90NoConcurrent1/2Unsuccessful*
16G3QHReiningR2Yes180YesConcurrent1/2Unsuccessful*
17G7WBJumpingL3Yes90NoMeniscus then SCL3Unsuccessful
18F4PaintHalterL4Yes700YesSCL then meniscus3Unsuccessful
19F8QHReiningR4Yes1460YesSCL then meniscus3Unsuccessful
20G7QHCuttingR3Yes700YesConcurrent1/2Successful
21F2QHNDR4Yes60NoSCL then meniscus3Euthanasia

All horses were lame at the time of the initial evaluation but the lameness grade was recorded for only 18/21 horses. The grade of recorded lameness range 2–4 with a mean of 2.9 and a median of 3. The duration of lameness was recorded for 17/21 horses, range 2 weeks–4 years, mean ± s.d. 272 ± 392 days and median 90 days. Fifteen horses had lesions of the right stifle and 6 involved the left.

Effusion of the MFT or femoropatellar joint was identified in 20 of the 21 horses. Thirteen horses had radiographic evidence of osteophytes of the MFT joint at initial evaluation and all but one had a reported duration of lameness of at least 4 months. Eight horses had no evidence of pre-existing osteophytes in the MFT joint. Radiographic evidence of osteophytes within the MFT joint was significantly associated with the duration of lameness as 8/8 horses that presented with lameness of >100 days' duration had radiographic evidence of osteophytes compared to 1/9 horses with a duration of lameness <100 days (P = 0.0004)

Thirteen horses had both medial meniscal and medial femoral condyle SCL that were identified concurrently at the initial arthroscopic examination (Fig 1). Ten horses with concurrent abnormalities had Grade 1/2 meniscal lesions and 3 had Grade 3 meniscal injuries. Eight of the horses with concurrently identified lesions had arthroscopic debridement of the SCL, 3 (all in the later years of the study when this technique became recommended) had intralesional steroid injections into the SCL, and the remaining 2 had no treatment, because the SCL could not be injected due to an articular subchondral bone plate covering the cyst opening. All meniscal lesions were debrided as much as possible arthroscopically. Two of these horses also developed SCL of the proximal medial aspect of the tibia on subsequent follow-up; one was debrided and the other was not treated.

Six horses had previous arthroscopic surgery of the MFT joint in which a SCL of the medial femoral condyle was debrided, and subsequently developed medial meniscal lesions (Fig 2). The meniscal lesions occurred at variable times after surgery but all were documented either with a second arthroscopy or at post mortem examination within one year of initial SCL debridement. Four of these horses developed Grade 3 meniscal lesions and 2 horses had Grade 1/2 meniscal injuries. The remaining 2 horses developed SCL of the medial femoral condyle subsequent to debridement of a primary medial meniscal injury. Both had Grade 3 meniscal tears at the initial surgery (Fig 3) and both SCL developed within 12 months of the initial surgery.

All meniscal lesions were identified in the medial meniscus. Twelve horses had a Grade 1/2 meniscal lesion (mild/moderate) and 9 Grade 3 (severe). The severity of the meniscal lesion appeared to be associated with the severity of lameness as 4/5 horses with a Grade 4 lameness had severe meniscal injuries, compared to 4/6 horses with Grade 3 and only 0/7 horses with a Grade 2 lameness had severe meniscal injuries (P = 0.008). However, the severity of meniscal injury was not associated with outcome even though all 4 horses that were successful had mild/moderate lesions.

The articular characteristics of the SCL were available for 19 of the 21 horses, as 2 horses developed SCL after debridement of a meniscal lesion and were not re-operated. The majority of these horses (n = 12) had the classic SCL appearance with a cartilage flap or dimpling at the entrance to a cloaca. The cystic cavities ranged in size from 1–3 cm in depth and were filled with fibrous tissue or yellow, granular bone. Five horses had evidence of full thickness cartilage erosions with soft subchondral bone beneath and 2 horses had a visible indentation within the cartilage and condyle but the SCL opening was bridged with bone.

Follow-up information was available for 19 out of 21 horses. Fifteen had an unsuccessful outcome (although 5 were being used for light riding) and only 4 returned to their intended use (Table 1). Six horses were re-examined by the authors and all were considered unsuccessful. Two of these horses had developed severe meniscal lesions following SCL debridement and were subjected to euthanasia due to severe lameness. Both horses that developed proximal medial tibial SCL remained lame and were retired to pasture. One horse that developed an SCL subsequent to debridement of a meniscus lesion remained lame at re-examination and was retired, and another had persistent lameness and was also retired to pasture. Two additional horses with Grade 3 meniscal tears and evidence of chronic osteoarthritis within the MFT joint were retired to breeding. Another horse was retired to pasture due to persistent lameness associated with the stifle joint and one horse was subjected to euthanasia after treatment due to the poor prognosis to return to soundness.

Four horses returned to their intended use; 2 with Grade 1/2 meniscal lesions subsequent to SCL debridement and 2 had Grade 1/2 meniscal lesions identified concurrently with an SCL. Five additional horses were being ridden but at a lower level than their intended use (Table 1). One of these horses developed an SCL following debridement of a Grade 3 meniscal tear and the other 4 had lesions that were identified concurrently. Overall, 9 of the 19 horses with follow-up (47%) were capable of some type of athletic activity. Treatment was successful in 4/10 geldings, 0/5 mares and 0/4 stallions (P = 0.12). Sex, age at presentation, type of lesion (concurrent, SCL then meniscus, or meniscus then SCL), duration of lameness, lameness severity, grade of meniscal injury and type of treatment used for the SCL did not appear to affect outcome.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Manufacturer's address
  8. References

This study reports a series of cases involving concurrent and/or sequential lesions of the meniscus/meniscal ligaments and medial femoral condyle. Until recently, curettage of the SCL has been the treatment of choice by many surgeons. Removal of the cystic lining was thought to result in filling in of the defect with more normal, healthy tissue. This mode of therapy has had mixed results with success rates ranging 64–82% in young horses (White et al. 1986; Howard et al. 1995; Sandler et al. 2002). However, removal of more than 15 mm of articular surface from the medial femoral condyle was shown to result in <30% chance to return to racing (Sandler et al. 2002). Smith et al. (2005) described that horses aged >3 years that underwent arthroscopic debridement of an SCL were <35% likely to return to their intended use. However, neither of these studies indicated whether subsequent or concurrent injury to the meniscus may have contributed to continued lameness and the poor prognosis for return to function. Although development of medial meniscal lesions following SCL debridement could be coincidental, it is also possible that removal of cartilage and bone overlying an SCL cavity may alter the contour of the medial femoral condyle, resulting in a more oblong surface. This alteration in contour of the medial femoral condyle may precipitate damage to the underlying meniscus; alternatively, the rim of bone at the edge of the debrided defect could act to damage the meniscus or cranial ligament of the medial meniscus. Six horses in this series developed subsequent meniscal injury following surgical debridement of an SCL. Two of these developed severe meniscal lesions that resulted in lameness that warranted euthanasia. In 2 additional horses, development of severe meniscal lesions resulted in retirement from performance. The remaining 2 horses developed only mild/moderate meniscal lesions and both returned to their intended use. Although uncommon, subsequent injury to the meniscus should be considered, particularly in those horses that remain lame or worsen after surgical debridement of an SCL.

Thirteen of 21 horses in this series had lesions of the meniscus and medial femoral condyle identified concurrently at the initial surgery; and 9 of these cases had radiographic evidence of osteophytes in the MFT joint upon initial evaluation, suggesting that these injuries may have occurred simultaneously from trauma to the joint. These findings also suggest that horses with radiographic evidence of osteophytes together with an SCL may potentially have a concurrent injury to the meniscus. Therefore, it would be appropriate to evaluate the medial meniscus using arthroscopy and/or ultrasound examination in horses with confirmed osteophytes of the MFT joint and an SCL visible on radiographs.

In this series, there were also 2 cases of acute lameness in which an SCL developed following debridement of a meniscal injury. Both were aged <3 years and in heavy work when the meniscal injuries occurred. No subchondral abnormalities were identified on initial radiographs or at arthroscopy. Both horses had meniscal injuries debrided with no visible damage to the medial femoral condyle, and both remained lame following surgery. On subsequent follow-up radiographs, SCL of the medial femoral condyle were identified. These findings suggest that concurrent bone and meniscal injury occurred in these horses at the time of initial injury, with a delay in the development of the SCL. It may be important to monitor horses with medial meniscal damage for later development of subchondral bone injury, particularly in those horses that fail to respond to surgery.

The median age of the horses in this study was 7 years, suggesting that the SCLs were probably more traumatic than developmental in origin. Age at presentation did not, however; appear to affect outcome. Only one of 7 horses aged ≤3 years were able to perform their intended use. This may be due to the combination of bone and meniscal involvement in horses in this study as compared to only SCL in younger horses in the study described by Smith et al. (2005). In general, the prognosis for horses with concurrent injuries within the MFT joint appears to be worse than for horses with either lesion alone.

The overall prognosis for horses with the combination of meniscal and SCLs within the MFT joint is guarded at best. Only 4 of 19 horses with follow-up returned to their intended use as performance horses, although an additional 5 were useable for light riding. All 4 horses that were successful had Grade 1/2 lesions of the medial meniscus; 2 developed meniscal lesions following debridement of an SCL; and 2 had a concurrent SCL and meniscal lesion. In addition, 4 of the 5 horses that were being used for light riding had Grade 1/2 meniscal injuries. This is consistent with the study by Walmsley (2003), which indicated a better prognosis for horses with Grade 1 and 2 lesions compared to horses with Grade 3 meniscal lesions. However, in the present study the severity of meniscal damage was significantly associated with the severity of lameness but not with outcome. This probably represents the lack of robustness of the statistical analysis in this study due to the small number of cases and the multiple variables associated with the outcome of clinical cases. Despite this limitation and the low occurrence of combined meniscal and subchondral bone injuries, the findings are important to clinicians when evaluating horses with stifle lameness.

In conclusion, this study supports the hypothesis, stated in previous studies, that trauma to the MFT joint can lead to subchondral injury and SCL formation (Ray et al. 1996). These findings also illustrate the importance of screening for both subchondral bone and/or meniscal lesions with the identification of either alone despite the low frequency of occurrence. Follow-up evaluation of horses undergoing SCL or meniscal debridement should be thorough and complete, utilising radiographic and ultrasound examination so that early identification of a subsequent lesion may be addressed and monitored. The authors also contend that surgical debridement of SCL should be performed with caution and possibly abandoned in favour of the injection technique (Goodrich and McIlwraith 2008; Wallis et al. 2008) to avoid the risk of subsequent meniscal injury. In addition, medial meniscal injuries should be investigated in horses with the combination of MFT joint osteophytes and SCL of the medial femoral condyle on stifle radiographs. Overall, horses that develop both an SCL and meniscal injury within the MFT joint have a guarded to poor prognosis to return to performance.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Manufacturer's address
  8. References
  • Goodrich, L.R. and McIlwraith, C.W. (2008) Subchondral bone cysts – not always an easy diagnosis. Equine vet. Educ. 20, 521-524.
  • Howard, R.D., McIlwraith, C.W. and Trotter, G.W. (1995) Arthroscopic surgery for chondral cystic lesions of the medial femoral condyle in horses: 41 cases (1988-1991). J. Am. vet. med Ass. 206, 842-850.
  • Jeffcott, L.B. and Kold, S.E. (1982a) Clinical and radiological aspects of stifle bone cysts in the horse. Equine vet. J. 14, 40-46.
  • Jeffcott, L.B. and Kold, S.E. (1982b) Stifle lameness in the horse: a survey of 86 referred cases. Equine vet. J. 14, 31-39.
  • Jeffcott, L.B., Kold, S.E. and Melsen, F. (1983) Aspects of the pathology of stifle bone cysts in the horse. Equine vet. J. 15, 304-311.
  • Kold, S.E. and Hickman, J. (1986) An experimental study of the healing process of equine chondral and osteochondral defects. Equine vet. J. 18, 18-24.
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  • McIlwraith, C.W. (1993) Inferences from referred clinical cases of osteochondrosis dissecans. Equine vet. J., Suppl. 16, 27-30.
  • McIlwraith, C.W., Nixon, A.J., Wright, I.M. and Boening, K.J. (2005) Diagnostic and Surgical Arthroscopy in the Horse, 3rd edn., Elsevier, Philadelphia. pp 197-268.
  • Ray, C.S., Baxter, G.M., McIlwraith, C.W., Trotter, G.W., Powers, B.E., Park, R.D., and Steyn, P.F. (1996) Development of subchondral cystic lesions after articular cartilage and subchondral bone damage in young horses. Equine vet. J. 28, 225-232.
  • Ross, M.W. and Dyson, S.J. (2003) Diagnosis and Management of Lameness in the Horse, W.B. Saunders, Philadelphia. pp 66-67.
  • Sandler, E.A., Bramlage, L.R., Embertson, R.M., Ruggles, A.J. and Frisbie, D.D. (2002) Correlation of lesion size with racing performance in thoroughbreds after arthroscopic treatment of subchondral cystic lesions of the medial femoral condyle. Proc. Am. Ass. equine Practnrs. 48, 255-256.
  • Semevolos, S.A. and Nixon, A.J. (2007) Osteochondrosis: Etiologic factors. Comp. cont. Educ. pract. Vet. 29, 158-164.
  • Scott, G.S.P., Crawford, W.H. and Colahan, P.T. (2004) Arthroscopic findings in horses with subtle radiographic evidence of osteochondral lesions of the medial femoral condyle: 15 cases (1995-2002). J. Am. vet. med. Ass. 224, 1821-1826.
  • Smith, M.A., Walmsley, J.P., Phillips, T.J., Pinchbeck, G.L., Booth, T.M., Greet, T.R.C., Richardson, D.W., Ross, M.W., Schramme, M.C., Singer, E.R., Smith, R.K. and Clegg, P.D. (2005) Effect of age at presentation on outcome following arthroscopic debridement of subchondral cystic lesions of the medial femoral condyle: 85 horses (1993-2003). Equine vet. J. 37, 175-180.
  • Trumble, T.N., Baxter, G.M., McIlwraith, C.W., Trotter, G.W. and Stashak, T.S. (1997) Arthroscopic treatment of ligamentous and meniscal injuries of the femorotibial joints in 29 horses. In: Proceedings of the American College of Veterinary Surgeons Symposium. p 102.
  • Verschooten, F. and DeMoor, A. (1982) Subchondral cystic and related lesions affecting the equine pedal bone and stifle. Equine vet. J. 14, 47-54.
  • Wallis, T.W., Goodrich, L.R., McIlwraith, C.W., Frisbie, D.D., Hendrickson D.A., Trotter, G.W., Baxter, G.M. and Kawcak, C.E. (2008) Arthroscopic injection of corticosteroids into the fibrous tissue of subchondral cystic lesions of the medial femoral condyle in horses: a retrospective study of 52 cases (2001-2006). Equine vet. J. 40, 461-467.
  • Walmsley, J.P. (1995) Vertical tears of the cranial horn of the meniscus and its cranial ligament in the equine femorotibial joint: 7 cases and their management by arthroscopic surgery. Equine vet. J. 27, 20-25.
  • Walmsley, J.P., Phillips, T.J. and Townsend, H.G.G. (2003) Meniscal tears in horses: an evaluation of clinical signs and arthroscopic treatment of 80 cases. Equine vet. J. 35, 402-406.
  • White, N.A., McIlwraith, C.W. and Allen D. (1986) Curettage of subchondral bone cysts in medial femoral condyles of the horse. Equine vet. J., Suppl. 6, 120-124.

Author contributions The initiation, conception and planning for this study were by S.M.H., G.M.B. and C.W.M. Its execution was by S.M.H., G.M.B., C.W.M., D.A.H., L.R.G. and G.W.T., with statistics by S.M.H. and D.D.F. The paper was written by S.M.H., G.M.B. and C.W.M. and L.R.G.