Equine Stifle Anatomy, Imaging, and Clinical Orthopedics

Anatomy and Soft Tissue Structures of the Equine Stifle

The equine stifle joint is a complex structure involving the distal femur, the tibia, and the patella. The menisci are fibrocartilaginous structures that increase the contact area between the femoral condyle and the tibial surface, effectively decreasing friction between the bones. Specifically, the cranial horn of the medial meniscus is under significantly more compressive force than other areas. During exercise, the shape of the menisci changes from a C-shape to an L-shape. The joint's stability is maintained by several leagues and soft tissue structures. These include the collateral ligaments (medial and lateral), the patellar ligaments (medial, middle, and lateral), the cruciate ligaments (cranial and caudal), and the intermeniscal, meniscofemoral, and meniscotibial (cranial and caudal) ligaments.

Stifle Joint Compartments and Communication

The stifle joint is divided into three primary compartments: the femoropatellar (FP), the medial femorotibial (MFT), and the lateral femorotibial (LFT). Injectable volumes for these joints are approximately $42\,ml$ for the medial femorotibial and $62\,ml$ for the lateral femorotibial compartments. Communication between these compartments is variable depending on the individual horse. Studies show that communication between the medial femorotibial and femoropatellar joints occurs in approximately $65\%$ of cases. The lateral femorotibial and femoropatellar joints communicate in $4\%$ to $25\%$ of cases, while the medial and lateral femorotibial joints communicate in only $8\%$ of horses. When using Carbocaine for anesthesia, however, there is essentially $100\%$ clinical communication or distribution accounted for across the joint.

Clinical Evaluation and Imaging Modalities

Evaluation of the stifle involves various imaging modalities. Radiographs and ultrasound are the primary tools used; computed tomography (CT) is difficult to utilize because the stifle is hard to fit into a standard CT scanner. It is important to note that foals have a different radiographic appearance than adults, which should not be confused with pathology. Standard radiographic views for the stifle include the Lateral, Cranial-caudal, $30^{\circ}$ Caudolateral-cranial medial oblique, and the Skyline view. The Lateral view is particularly useful for visualizing the lateral and medial trochlear ridges.

Osteoarthritis (OA) of the Stifle

Osteoarthritis (OA) in the stifle presents clinically as lameness and a positive response to upper limb flexion. Effusion may or may not be present. Localization is achieved through intra-articular anesthesia. Radiographic abnormalities associated with OA include decreased joint space, osteophyte formation on the medial tibial plateau, and flattening of the medial femoral condyle. Treatment typically involves biologics or intra-articular injections, with a standard protocol being a combination of $5\,mg$ of triamcinolone and $3\,ml$ of polyglycan HV.

Osteochondritis Dissecans (OCD) of the Lateral Trochlear Ridge

Osteochondritis Dissecans (OCD) lesions often manifest as lateral trochlear ridge fragments, causing lameness accompanied by effusion. Small lesions located distally generally carry a better prognosis. Surgical treatment involves arthroscopic removal of fragments or debridement. Current research into future treatments includes the re-attachment of fragments or allowing for a fibrocartilage scar to form. The prognosis for return to performance is directly related to the size of the lesion: lesions $< 2\,cm$ have a $78\%$ success rate; lesions between $2\,cm$ and $4\,cm$ have a $63\%$ success rate; and lesions $> 4\,cm$ have a $54\%$ success rate.

Subchondral Bone Cysts (SBC)

Subchondral bone cysts (SBC) are commonly found in the medial femoral condyle. While they cause lameness, joint effusion may be absent. Histologically, these cysts contain fibroplasia, degenerate bone and cartilage, granulation tissue, and woven or immature bone. Santschi et al. (2014) established a grading scale for SBC: Grade 1 involves flattening; Grade 2 is a dome-shaped lucency $< 10\,mm$; Grade 3 is a dome-shaped lucency with no cloaca; Grade 4 is a dome-shaped lucency $> 10\,mm$ that communicates with the joint; Grade 5 is a dome-shaped lucency > 10\,mm with a narrow cloaca; and Grade 6 includes grade 4 or 5 with additional lesions. Sclerosis is often noted around these lucencies. Treatment can be conservative (reduced exercise and intra-articular steroid injections) or surgical (arthroscopic debridement, osteochondral grafts, transcortical screws, or the injection of bone substitutes, growth factors, or stem cells). The prognosis is size-dependent: cysts $< 15\,mm$ in diameter have a $70\%$ return-to-performance rate, while those $> 15\,mm$ have only a $30\%$ return-to-performance rate.

Meniscal and Soft Tissue Injuries

Meniscal injuries can be primary, caused by sudden concussive forces, or secondary, resulting from supporting ligamentous tears. These are notoriously hard to diagnose radiographically; clinicians should look for calcification resulting from chronic damage or evidence of arthritis, such as fragments at the cranial attachment of the medial meniscus. Ultrasound or arthroscopy are often needed to identify the fraying of ligaments or menisci. Damage is graded on a scale of 1 to 3, which dictates the prognosis: Grade 1 has a $63\%$ return-to-function rate; Grade 2 has a $56\%$ rate; and Grade 3 has a very poor prognosis of only $6\%$. Treatment options include conservative management or arthroscopic debridement.

Disorders of the Patella: Upward Fixation and Lateral Luxation

Upward Fixation of the Patella occurs when the horse cannot release the passive stay apparatus of the stifle, often due to weak quadriceps. It is common in young horses and presents as a straight hindlimb appearance, sometimes resulting in "pointy toes." The fixation can be intermittent or permanent. To unlock the patella, the horse may need to be backed up. Treatment involves strengthening the quadriceps or surgical interventions such as a medial patellar ligament desmotomy or splitting.

Lateral Luxation of the Patella is typically seen in foals and miniature horses and can be unilateral or bilateral. Complete luxation disables the quadriceps, preventing the horse from locking its legs. Surgical correction is required, involving a lateral release with medial imbrication and sulcoplasty, similar to procedures performed in canine patients.