The Patella and Femoropatellar Joint
The patella and patella ligaments are a location of muscular insertion as well as an important component of the ability for the stifle to “lock”, allowing one hind limb to rest while the other supports most of the body weight. The first division of biceps femoris inserts onto the patella and the second division onto the lateral patella ligament [Figure 1], the four heads of the quadriceps femoris also combine and inset together onto the patella and intermediate patella ligament, the medial patella ligament runs from the parapatellar cartilage and inserts medially to the intermediate ligament at the tibial tuberosity (Dyce et al., 2010). It may also be considered that the patella is a sesamoid bone in the insertion of Quadriceps Femoris and the patella ligaments are tendons of insertion of quadriceps femoris and biceps femoris onto the tibial tuberosity (Hermans et al., 1987). The patella sits cranially to the femoral trochlea [Figure 2] which is composed of asymmetrical medial and lateral ridges with the medial being larger and extended proximally – visible between the medial and intermediate patella ligaments. The cranial surface of the trochlear is the gliding surface, and above this is a narrow shelf forming the resting surface. The largest caudal surface of the patella articulates with the trochlea during most of the normal range of motion, and a narrow articular surface at the patella’s apex contacts the femur at the limit of extension (Dyce et al., 2010). The patella appears to move proximally when the stifle is in extension and distally during flexion, the femur also moves during motion changing the position of the trochlea regarding the patella (Hermans et al., 1987).
Figure 2. Lateromedial radiograph of the equine patella in its correct anatomical position (Hall et al., 2010).
References
ANDERSEN, C. & TNIBAR, A. 2016. Medial patellar ligament splitting in horses with upward fixation of the patella: A long-term follow-up. Equine Vet J, 48, 312-4.
BAXTER, G. M. 2011. Manual of Equine Lameness, UK, Wiley-Blackwell.
DYCE, K. M., SACK, W. O. & WENSING, C. J. G. 2010. Textbook of Veterinary Anatomy, Missouri, Saunders.
ENGELBERT, T. A., TATE, L. P., RICHARDSON, D. C., HONORE, E. K. & LITTLE, E. D. E. 1993. Lateral Patellar Luxation in Miniature Horses. Veterinary Surgery, 22, 293-297.
HALL, M. S., JALIM, S. L. & RUSSELL, T. M. 2010. Distal luxation of the patella in a horse. Australian Veterinary Journal, 88, 396-398.
HERMANS, W. A., KERSJES, A. W., VAN DER MEY, G. J. & DIK, K. J. 1987. Investigation into the heredity of congenital lateral patellar (sub)luxation in the Shetland pony. Vet Q, 9, 1-8.
LACROIX, J. V. 2005. Lameness of the Horse, The Project Gutenberg eBook.
O'MEARA, B. & LISCHER, C. J. 2009. Surgical management of a pony with a traumatic medial luxation of the patella. Equine Veterinary Education, 21, 458-463.
PEITZMEIER, M. D., KOONTZ, Z. D., LYNCH, T. M., HUGHES, F. E. & SLONE, D. E. 2015. Outcome of medial patellar ligament desmoplasty for treatment of intermittent upward fixation of the patella in 24 horses (2005–2012). The Canadian Veterinary Journal, 56, 193-195.
ROSS, W. M. & DYSON, J. S. 2003. Diagnosis and Management of Lameness in the Horse, Missouri, Saunders.
RRBARRANCH. 2016, Upward fixation of the patella, Youtube, 29/10/2016, viewed 31/07/17, https://www.youtube.com/watch?v=_f1cCnmd4YY.
SKARRILD, L. 2011, Horse with patella luxation, Youtube, 15/11/2011, viewed 31/07/17, https://www.youtube.com/watch?v=RBQ2xJM3a-8.
TNIBAR, A. 2003. Treatment of upward fixation of the patella in the horse: an update. Equine Veterinary Education, 15, 236-242.
Figure 1. An illustration showing the dorsal view of the right equine stifle, demonstrating the patella ligaments and associated structures. The belly of the long digital extensor is removed (Baxter, 2011).
Upward Fixation of the Patella
Clinical Signs & Diagnosis
- Foals that are severely affected cannot extend the stifle and so stand in a crouching position (Ross and Dyson, 2003).
- In less severe cases horses will usually be unwilling to flex the stifle, and the gait is stiff in the affected hindquarter (Ross and Dyson, 2003).
- The condition may not appear until degenerative changes cause lameness, e.g. osteochondrosis of a trochlea ridge (Ross and Dyson, 2003).
- The severity is graded, grade 1: the patella can easily reduce itself after manual luxation. Grade 2: the patella intermittently luxates from the trochlea groove. Grade 3: the patella can be manually reduced although it is usually luxated. Grade 4: the patella cannot be manually reduced from the luxated position (Engelbert et al., 1993).
- Can be diagnosed by palpation and examination of radiography. Useful radiographic views are latero-medial and caudoproximal-craniodistal oblique of the stifle joint and a cranioproximal-craniodistal skyline of the patella, the last being particularly useful to assess the trochlea ridges and groove and patella if a congenital condition is anticipated (O'Meara and Lischer, 2009).
- Lateral release of the patella can be achieved by incising the fascia lata and fibrous joint capsule located lateral to the lateral patella ligament, incision originates at the fascia lata insertion and continues cranio-proximally until the patella can be repositioned and sustains correct position through flexion and extension of the stifle (Engelbert et al., 1993).
- The patella is stabilised by medial imbrication of the supporting structures of the stifle. The tendon of sartorius muscle and parapatellar fascia can be imbricated to the joint capsule and medial patella ligament (Engelbert et al., 1993).
- To surgically repair a medial luxation it is possible to release the patella by transection of the medial femoropatellar ligament, then replacing the lateral femoropatellar ligament using a prosthetic suture (anchored by intra-patellar drill holes) and reinforcing the lateral retinaculum using mesh implant (anchored by cortical screws) on the lateral side of the joint (O'Meara and Lischer, 2009).
- Prognosis is poor in larger horses and in the case of osteoarthritis, but there is sufficient potential for foals to attain athletic function (Ross and Dyson, 2003).
- Distal luxation can be corrected by manual manipulation under general anaesthesia (Hall et al., 2010).
Treatment & Prognosis
Upward fixation of the patella occurs when the stifle forms an angle of approximately 145 degrees (Ross and Dyson, 2003) where the medial patella ligament passes over the medial trochlear ridge and is fixed there (Baxter, 2011), the hindlimb is locked in extension. Delayed release of the patella does not include complete upward fixation and so is a less severe form (Ross and Dyson, 2003). Upward fixation of the patella is most commonly seen in ponies and young horses. Horses may be prone to upward fixation of the patella if they have straight hindlimb conformation as a smaller degree of extension is required for fixation to occur (Ross and Dyson, 2003). Loss of muscle tone in the quadriceps results in the medial patella ligament lengthening, making it more prone to catching over the medial trochlear ridge (Baxter, 2011). Elongated toes and tall medial hoof walls (Peitzmeier et al., 2015) is another proposed predisposition.
- The horse will stand with the hindlimb locked in extension and the fetlock joint flexed (Ross and Dyson, 2003) [Figure 3].
- In acute cases the horse will drag the dorsal surface of the hoof along the ground when asked to move forward (Baxter, 2011).
- The horse is usually able to unlock the joint on its own, but may need assistance (Ross and Dyson, 2003). A clicking sound may be heard as the patella is released.
- Intermittent upward fixation of the patella can occur and is most obvious when walking a horse downhill, backing up, or moving in a tight circle. This may be mistaken for stringhalt, as when the medial patella ligament releases the hindlimb will swiftly jerk up (Baxter, 2011).
- Usually both hindlimbs are affected (Baxter, 2011).
- Stifle soreness will result if the condition is chronic, and in serious cases horses will try to avoid extending the limb while walking up or downhill (Ross and Dyson, 2003).
Clinical Signs & Diagnosis
Treatment & Prognosis
- Simultaneous stifle disease should be validated by diagnostic analgesia, and x-rays of both stifles examined to rule out conditions caused by the upward fixing patella which would affect treatment and prognosis (Ross and Dyson, 2003).
- Persistent upward fixation of the patella frequently occurring after manual unlocking usually requires splitting of the medial patella ligament (desmoplasty) which is minimally invasive and highly effective, allowing quick return to physical activity (Andersen and Tnibar, 2016).
- In milder cases horses can undergo quadriceps strength and conditioning to tighten the medial patella ligament, such as walking up hills, horses that respond to the program and maintain fitness have good prognosis (Baxter, 2011).
- Corrective trimming to shorten the toe length and lower the medial hoof wall may be sufficient in some cases (Baxter, 2011).
- Other medical therapies include injection of counterirritants (thickens the medial patella ligament (Baxter, 2011)), joint supplements, anabolic steroids, chondro-protective joint injections and estrogen therapy (Peitzmeier et al., 2015).
Figure 3. An equine hindlimb, showing the typical posture of upward fixation of the patella (Tnibar, 2003).
Patella Luxation
Luxation of the patella occurs when the patella is dislocated from its normal position over the groove of the femoral trochlea displacing the patella to outside of the femoropatellar joint, during motion or at rest (Hermans et al., 1987). Due to the larger and proximally extended medial trochlear ridge most cases of patella displacement are lateral, medial displacement of the patella is only evident in cases of severe trauma (Ross and Dyson, 2003). In general, patella luxation can have a developmental, congenital or traumatic origin and can occur in one or both hindlimbs (O'Meara and Lischer, 2009). Distal luxation of the patella [Figure 4] may occur in a case of poor muscle mass and body condition, therefore poor quadriceps tone. A horse in this condition may lose its footing, resulting caudal extension of the hindlegs can cause hyperflexion of the stifle joint and over-extension of the quadriceps muscle, allowing the patella to become distally positioned (Hall et al., 2010). Lateral patella luxation (usually seen in foals) may be due to ligamentous, muscular and osseous defects, most cases appear to be hereditary and may be a recessive trait (Engelbert et al., 1993). Luxation of the patella is rarely seen in adult horses and usually originates from trauma (O'Meara and Lischer, 2009), it is most often observed in miniature breeds (Ross and Dyson, 2003). The luxation may be temporary and correct itself, or it may become fixed and require remedial manipulation (Lacroix, 2005).
Figure 2. Lateromedial radiograph of the equine patella in its correct anatomical position (Hall et al., 2010).
Figure 4. Lateromedial radiograph exhibiting distal patella luxation in the horse (Hall et al., 2010).
(Rrbarranch, 2016)
(Skarrild, 2011)