The patellar ligaments are a functional continuation of the quadriceps and biceps femoris muscles, the major extensors of the stifle, originating on the patella and inserting on the cranioproximal aspect of the tibia. The middle patellar ligament is the largest of the ligaments and is circular to oval in cross-section. The patellar ligaments are extra-articular, but have a close anatomical relationship with the femoropatellar joint capsule on their caudal aspect and overlying fat cranially. The middle patellar ligament has a well-organised linear fibre pattern in longitudinal ultrasonographic images (Dyson 2002). It blends with the smooth osseous contours of the patella and tibia proximally and distally, respectively. In transverse images the middle patellar ligament is fairly uniform in echogenicity. However, there may be radiating relatively hypoechoic lines in some horses. In horses which are excessively fat, ultrasonographic assessment of the patellar ligaments can be challenging. Radiopharmaceutical uptake (RU) in the patella and the cranioproximal aspect of the tibia are rather variable in normal mature horses (Dyson et al. 2007). Comparing ratios of RU with a standard site in the diaphysis of the femur, the ratio for the patella usually exceeds that of the cranioproximal aspect of the tibia.
Diagnosis of injuries of the middle patellar ligament is challenging, unless the injury is severe. There may be no localising clinical signs and no response to intra-articular analgesia of the stifle. The degree of associated lameness varies markedly, but is generally related to the severity of injury. In some horses there is mild to moderate improvement in lameness after intra-articular analgesia of the femoropatellar joint. However, with severe injuries there may be extensive periligamentous soft tissue swelling, with or without distension of the femoropatellar joint capsule. With severe lesions, torn fibres may be herniated into the femoropatellar joint. Injuries may occur anywhere along the length of the ligament, usually involving the origin, mid body or insertion. Previous desmotomy of the medial patellar ligament may be a predisposing factor for injury of the middle patellar ligament in some horses (Dyson 2002); medial patellar ligament desmotomy invariably results in chronic enlargement of the ligament which is readily palpable. Primary injuries of the medial and lateral patellar ligaments are less common than middle patellar ligament injury.
Ultrasonographic assessment of the patellar ligaments is recommended in any horse which has lameness which is not altered by local analgesic techniques, or in which there is partial improvement in lameness after intra-articular analgesia of the femoropatellar joint. It is also recommended if there is diffuse soft tissue swelling on the cranial aspect of the stifle. Lesions of the middle patellar ligament are characterised by enlargement of the ligament, focal or diffuse areas of reduction in echogenicity, loss of fibre pattern periligamentous echogenic material. It should be borne in mind that lameness associated with previous injuries can resolve, despite the persistence of hypoechoic regions in the middle patellar ligament, which may persist long-term. Lesions at the origin or insertion may have associated irregularities of the osseous contour of the patella or tibia, respectively.
Occasionally entheseous new bone has been identified radiologically on the distal aspect of the patella, but often there are no associated radiological abnormalities. In some horses there is focal intense increased RU (IRU) in the cranioproximal aspect of the tibia in a lateral scintigraphic image in association with patellar ligament injury. However, the absence of IRU does not preclude patellar ligament injury and focal IRU has also been seen as an incidental abnormality in horses with pain causing lameness localised to another site.
Horses with severe injuries of the middle patellar ligament, with extensive periligamentar reaction, have generally remained lame irrespective of treatment. Horses with less severe injuries have responded variably to conservative management of rest alone (minimum 6 months), with a large proportion showing residual long-term lameness. Better results have been achieved by intralesional injection of porcine bladder matrix or platelet rich plasma, with some horses returning to full athletic function.