Treatment of Subchondral Cystic Lesions of the Stifle

Take Home Message: There are various manifestations of subchondral cystic lesions (SCLs). Treatment of clinically significant lesions revolves around Type 1 and Type 2 lesions. A consensus algorithm for treatment of SCLs of the medial femoral condyle (MFC) involves arthroscopic surgery and if the cyst has stable margins to a probe, and there is no penetration or collapse of the margins, intralesional corticosteroid injection is the first option. Cysts with collapsed margins are debrided. Failure to respond to intralesional corticosteroids leads to a recommendation of arthroscopic evacuation and debridement in an immature horse and arthroscopic debridement plus augmentation in an older horse. A recent option includes placement of a cortical bone screw across the lesion.

I. INTRODUCTION

Subchondral cystic lesions and their pathogenesis and consequent treatment methods in particular, still incite controversy.¹ The lesions were initially described as subchondral bone cysts; later, other authors described them as subchondral cystic lesions or osseous cyst-like lesions to avoid the implication that they are a true cyst. However, when examined pathologically, the author feels that they conform to most people’s definition of a cyst in that they have a lining (Fig. 1). They occur in a number of locations. Subchondral bone cysts were first reported as a clinical entity in 1968.² In that report there were 12 cases in the phalanges and one in the radial carpal bone. A series was reported in 1982 in the stifle and distal phalanx.³ A third series of 69 cases with 64 horses was reported in 1970 under a modified name, osseous cyst-like lesions.⁴ In that series, there were 15 instances of cysts in the carpal bones, 10 in the third metacarpal bone, 3 in the radius, 5 in the proximal sesamoid, 6 in the proximal phalanx, 4 in the middle phalanx, 5 in the distal phalanx, 6 in the navicular bone, 12 in the femur, 2 in the tibia and 3 in the tarsal bones. Since that time, the most common site of clinical cases reported has been the medial condyle of the femur.^5-12
 

II. WHAT IS A SUBCHONDRAL CYSTIC LESION AND WHAT ARE WE TREATING?

This discussion will address SCLs of the MFC in the medial femorotibial joint of the stifle as this is the most common condition we deal with and studies on pathogenesis have been specific for this lesion.

In early literature, cases of SCLs had lameness and radiographs usually showing an obvious lesion. Lameness is the usual reason that cases are presented to the veterinarian. Direct palpation of medial femorotibial joint effusion is uncommon but it has been reported that approximately 60% of cases will report with femoropatellar effusion.⁵ This is presumably related to known communication (at least the potential) between the medial femorotibial joint and the femoropatellar joint. The condition is confirmed by radiographs (Fig. 2). With the advent of digital radiographs and survey radiographs at yearling sales, more attention is now paid to subchondral defects and even flattening of the medial femoral condyle in addition to ‘traditional’ SCLs. These cases are usually asymptomatic.
 

With regard to radiographic appearance and classification, there has been an evolution from initially two types: Type I being a radiographically dome shaped lucent area which was confluent with a flattened joint surface and Type II had a circular lucent area within the condyle with a thinner radiographically lucent tract connecting the cysts to the articular surface of the condyle (Fig. 2).¹² This early classification evolved into three types of SCLs. Type I - 10mm or less in depth, Type II - more than 10mm in depth and Type III - flattened or irregular contour of the subchrondal bone.⁵ Morerecently,five different types have been described by Wallis et al (Fig. 3).¹³ Type 1 lesions were defined as being <10mm in depth and were usually dome shaped. Type 2A lesions were >10mm in depth and had a lollipop or mushroom shape with a narrow cloaca and a round cystic lucency. Type 2B lesions were >10mm in depth with a large dome shape extending down to a large articular surface defect. Type 3 lesions (noted incidentally on survey radiographs of yearlings) were defined as condylar flattening or small defects in the subchondral bone, usually noted in the contralateral limb to that of the clinically significant SCL. Type 4 lesions were defined as those that had lucency in the condyle with or without an articular defect, but had no radiographic evidence of a cloaca in the subchondral bone plate.

Digital radiographs and survey radiographs have led to increased scrutiny of radiographs. As part of a study to prospectively examine the significance of lesions in the medial femoral condyle of cutting horses there was further definition of subchondral defects and flattening of MFCs.¹⁴ In this publication to differentiate ‘lesions’ that were being noted as potentially significant by some veterinarians, the MFC was classified as smooth and continuously convex in contour (Grade 0); flattened without radiographic evidence of subchondral bone changes (Grade 1); a small defect or change in, without extension through, the subchondral bone (Grade 2); a shallow, crescent shaped subchondral lucency that is wider than tall and confluent with the joint surface (Grade 3); or a well-defined round or oval radiolucency in the middle of the MFC that communicated with the femorotibial joint (Grade 4)¹⁴ (In this classification Grade 3 is equivalent to the Type 1 of Wallis et al¹³ and Grade 4 equivalent to Grade 2 without subdivision into a and b).
 

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