Chapter 56Diagnosis and Management of Osteochondrosis and Osseous Cystlike Lesions

The pathogenesis of osteochondrosis (OC), palmar or plantar osteochondral fragments of the proximal phalanx, ununited palmar or plantar eminences of the proximal phalanx, subchondral bone cysts, and other osseous cystlike lesions is discussed in Chapters 36, 42, and 54. In this book the term subchondral bone cyst is used for large radiolucent areas in the subchondral bone of the medial femoral condyle; these cysts have a consistent pathological structure with a true cellular cyst lining. Osseous cystlike lesions in other locations vary more in structure. This chapter discusses the diagnosis and management of these lesions and other osseous fragments believed to be at least predominantly developmental in origin. The joints most commonly affected include the hock, stifle, fetlock, and shoulder. It is common for matching joints to have lesions but much less common to find OC lesions in different joints. This is consistent with the concept of such lesions occurring during some finite window of vulnerability during development that is specific for a given anatomical location.

Clinical signs of OC range from none to severe lameness. Moderate-to-severe joint effusion often occurs with minimal or no lameness. The clinical signs are not necessarily coincident with the development of the lesion. Most OC lesions are probably formed before 6 months of age, yet most are diagnosed at a later age. Some lesions seen radiologically heal, or they at least become stable enough never to cause a clinically apparent lameness. Lesions in racing Thoroughbreds (TBs) and Standardbreds (STBs) are usually recognized by 2 years of age, but in Warmbloods (WBLs) that are older when they begin training, clinical signs may not be seen until the horse is 5 to 6 years of age or even older. Acute, more severe lameness in older horses occurs occasionally when osteochondritis dissecans (OCD) fragments become loose.

Diagnosis is based on typical clinical signs and radiology. Most OCD flaps in horses have an osseous component and so can be identified using plain radiography. Defects in subchondral bone contours are also easily identified. Radiologically apparent defects in subchondral bone must be interpreted with caution in foals, especially in areas where endochondral ossification occurs later, such as the femoral or talar trochlear ridges and the third metacarpal or metatarsal sagittal ridge. An irregularity in the contour of subchondral bone does not mean that an articular defect is necessarily present; the lesion still possibly may heal. Therefore surgical intervention should be delayed until it is clear that either a loose fragment is present or the healing is progressing inadequately. Radiologically apparent lesions must always be interpreted with the clinical signs, because some do not cause clinical signs. If a lesion is identified, the contralateral joint should always be examined radiologically, because bilateral involvement (quadrilateral in the fetlock) is common. The presence of bilateral radiologically apparent lesions in a horse that is unilaterally lame is not unusual and further emphasizes the need to clinically evaluate the individual horse fully.

Decisions concerning treatment should take into consideration the age of the horse, its intended use, the severity of the lesions and anatomical location, whether the horse is intended for sale through public auction, and, if so, the timing of the sale and the conditions of sale. Prophylactic removal of OCD lesions for economic reasons may be justified in all horses intended for future sale. Most buyers are more willing to purchase horses after an OCD lesion has been removed and the horse has returned to work.

Lesions in the Metacarpophalangeal and Metatarsophalangeal Joints

Osteochondritis Dissecans of the Sagittal Ridge of the Third Metacarpal and Metatarsal Bones

Most foals and yearlings with OCD lesions of the sagittal ridge of the third metacarpal bone (McIII) or third metatarsal bone (MtIII) are not recognized as lame. A variable degree of joint effusion occurs, and the lesions are usually first identified on presale radiographs. Unstable lesions cause lameness and persistent effusion when a horse starts work. Lameness is eliminated by intraarticular or perineural analgesia.

Lesions are more common in the MtIII than in the McIII, but all fetlocks should be examined radiologically if a lesion is recognized in one, using perfectly exposed (or slightly underexposed) flexed lateromedial images (Figure 56-1). Lesions on the dorsoproximal aspect of the sagittal ridge are easiest to recognize, but lesions may occur farther distally or, less commonly, on the palmar or plantar aspect of the sagittal ridge. The latter can also be seen radiologically on the dorsopalmar (plantar) image. The larger fragments extend abaxially along the proximal margin of the articular cartilage under the dorsal synovial fold.

Fig. 56-1 A typical distal sagittal ridge osteochondritis dissecans lesion (arrows) of the third metacarpal bone is seen best on a slightly underexposed flexed lateromedial radiographic image (A) and a well-exposed dorsopalmar image (B).

Conservative management should always be considered in horses younger than 18 months old, because lesions can show improvement radiologically and presumably heal. Surgery is indicated in young horses with fetlock effusion and lameness. Young horses treated surgically often do well clinically, but the long-term radiological appearance is inferior to that seen in horses in which the lesions heal spontaneously. Older horses of training age with unstable fragments are treated by arthroscopic debridement. Proximal lesions are technically easy to approach and debride, but lesions that are more distal can be exposed only by slight or moderate flexion of the joint. The arthroscope portal therefore should be made more distal than usual, and a needle should be used to identify a suitable instrument portal. Debriding a sagittal ridge lesion with a portal placed through the extensor tendon directly over the lesion is easiest. Long reaches across a flexed joint may result in undesirable iatrogenic trauma.

The prognosis depends on the size and location of the flap. Probably 90% of horses with typical lesions of the more proximal portion of the sagittal ridge go on to athletic function. Horses with distal lesions in a more weight-bearing location do not do as well as those with flaps at the proximal articular margin, and the radiological appearance does not improve spontaneously as often.

Osseous Cystlike Lesions

Osseous cystlike lesions occur on the weight-bearing surface of the condyles of the McIII and the MtIII. Most occur in the medial condyle and are diagnosed in horses 1 to 2 years of age. The lesions are easy to recognize radiologically (Figure 56-2) and result in obvious lameness, exacerbated by distal limb flexion. Lameness is improved by perineural or intraarticular analgesia. Lameness associated with an osseous cystlike lesion sometimes occurs in foals. At the time of onset of lameness no lesion may be identifiable radiologically, or only a subtle defect in the outline of the subchondral bone may be apparent. However, sequential radiographs obtained over the following 6 to 8 weeks may reveal development of an osseous cystlike lesion. Surgical debridement may be successful,¹ but corticosteroids administered intraarticularly or intralesionally also may result in clinically significant improvement in young horses. Although simple debridement of the McIII and the MtIII osseous cystlike lesions has been reasonably successful, the intensity of loading on the distal aspect of the McIII and the MtIII in athletic horses makes horses with these lesions good candidates for advanced joint resurfacing techniques.