Medial patellar ligament desmotomy and patellar fragmentation in horses

Medial patellar ligament desmotomy is a surgical procedure used to treat upward fixation of the patella, commonly referred to as locking stifle. One notable complication of this procedure is fragmentation or chondromalacia of the distal patella.¹

Anatomy and pathophysiology

Unlike humans and other species, horses possess 3 ligaments that secure the patella and reinforce the synovial capsule of the femoropatellar joint: the medial, middle, and lateral patellar ligaments.² Upward fixation of the patella occurs when the medial patellar ligament becomes trapped over the medial femoral trochlear ridge, resulting in the locking of the reciprocal apparatus in extension.^2,3 Various medical and surgical treatments have been explored for this condition.^2-4

Medical management

Medical approaches involve avoiding confinement, utilizing physiotherapy to strengthen the quadriceps muscles (which attach to the patellar ligaments),^2,3 and optimizing hoof balance by shortening the toe and trimming the medial hoof wall to facilitate medial breakover.³ These measures are beneficial in selected cases. Injection of irritant solutions into the medial patellar ligament can also be performed in standing horses to induce inflammation and alter ligament size and shape.⁵

Surgical management

Surgical options include desmoplasty, or splitting, of the medial patellar ligament, in which longitudinal incisions are made to induce irritation and contraction, resulting in functional shortening and tightening of the ligament.⁶ In a 2014 study, patellar ligament desmoplasty completely resolved the condition in 58% of horses and improved it in an additional 31%, with resolution observed in 60 to 90 days, and 73% reaching the desired performance level.⁷ Complications, such as ligament rupture, occurred in 1 of 64 horses (~1.6%).⁷ In a 2015 report, although 71% of horses returned to exercise after desmoplasty, only 18% returned to a higher performance level, and 50% of owners were satisfied.⁸

Medial patellar ligament desmotomy involves complete transection of the ligament and can be performed with the horse standing under sedation or under general anesthesia. A 2004 study showed a 98% resolution rate, with 94% of horses sound postoperatively. Six horses developed lameness that resolved with rest and medical management, although in 2 cases it persisted for 5 to 6 months.⁹

Complication: Patellar fragmentation

Fragmentation of the apex of the patella is a recognized complication of medial patellar ligament desmotomy.¹ However, similar lesions can also result from trauma or repetitive use.^3,10,11 Fragmentation is believed to reflect degenerative changes—rather than true fracture—caused by altered patellar positioning and increased stress on the remaining patellar ligaments, particularly the middle patellar ligament.^4,11 The risk may increase if horses return to work too soon after surgery, suggesting that adequate postoperative rest could help reduce risk.^2,4 Nonetheless, some studies indicate that even a 120-day rest period does not necessarily prevent postsurgical instability and related patellar changes.¹² Because of this complication, desmotomy of the medial patellar ligament should not be considered the treatment of choice.

Diagnosing patellar fragmentation

Diagnosis relies on clinical history, musculoskeletal and lameness examinations, and diagnostic imaging. A history of medial patellar ligament desmotomy is useful, but not always available. Clinical signs may include mild to severe lameness, fibrosis at the surgical site, and femoropatellar joint effusion.^3,10 Radiography has low sensitivity for detecting cartilage lesions but can be supplemented with ultrasonography.^13,14 Nuclear scintigraphy is sensitive but not specific, while arthroscopy remains the gold standard in humans for diagnosing and grading patellar chondromalacia.¹ Lesions may range from distinct fragmentation (Figure 1) or osteophyte formation to milder fibrillation or lysis of the patellar apex.¹¹

Treatment

Surgical treatment consists of femoropatellar joint arthroscopy under general anesthesia, allowing direct examination of the lesion, which may present as cartilage fibrillation, softening, dissecting lesions, or osteochondral fragmentation.¹⁵ Treatment involves debridement of abnormal cartilage and subchondral bone, followed by thorough joint lavage.¹⁰ (Figure 2) Postoperative care emphasizes rest and gradual return to exercise. Intra-articular biologic injections are recommended by the author; however, efficacy for this specific condition has not been thoroughly evaluated.

Prognosis

Patellar fragmentation occurs less frequently today, likely due to the availability of alternative treatments for upward fixation of the patella.^2,10 Evidence regarding this complication is limited, but the prognosis is generally favorable. A 1990 report found that 80% of 15 horses with distal patellar fragmentation had a history of medial patellar ligament desmotomy, of which 66% returned to their intended use. Among the 3 horses without prior desmotomy, 2 also returned to their intended use.¹⁶

Summary

A stepwise approach to upward fixation of the patella—beginning with conservative measures and progressing to surgery as indicated—is recommended based on severity. Although medial patellar ligament desmotomy can be effective, potential complications such as patellar fragmentation should be discussed with owners. With accurate diagnosis and appropriate surgical management, patellar chondromalacia generally carries a good prognosis, and most horses return to their intended use. Supportive therapies such as intra-articular orthobiologics postoperatively may further optimize outcomes.

References

1. McLellan J, Plevin S, Hammock PD, BonenClark G. Comparison of radiography, scintigraphy and ultrasonography in the diagnosis of patellar chondromalacia in a horse, confirmed by arthroscopy. Equine Vet Educ. 2009;21(12):642-647. https://doi.org/10.2746/095777309X474314
2. Ross MW, Dyson SJ, eds. Diagnosis and Management of Lameness in the Horse. 2nd ed. Elsevier Saunders; 2010.
3. Baxter GM, ed. Adams and Stashak’s Lameness in Horses. 7th ed. Wiley-Blackwell; 2020.
4. Auer JA, Stick JA, Kümmerle JM, Prange T, eds. Equine Surgery. 5th ed. Elsevier; 2019.
5. Brown MP, Moon PD, Buergelt CD. The effects of injection of an iodine counterirritant into the patellar ligaments of ponies: application to stifle lameness. J Equine Vet Sci. 1984;4(2):82-87. doi:10.1016/S07-0806(84)80087-8
6. Reiners SR, May K, DiGrassie W, Moore T. How to perform a standing medial patellar ligament splitting. Proc Am Assoc Equine Pract. December 7, 2005. Accessed June 11, 2026. https://www.ivis.org/library/aaep/aaep-annual-convention-seattle-2005/how-to-perform-a-standing-medial-patellar-ligament-splitting
7. James SJ, Eastman TG, McCormick JD. Long-term outcome of standing medial patellar ligament splitting to manage horses exhibiting delayed patellar release: 64 horses. J Equine Vet Sci. 2014;34(4):479-483. doi:10.1016/j.jevs.2013.09.011
8. Peitzmeier MD, Koontz ZD, Lynch TM, Hughes FE, Slone DE. Outcome of medial patellar ligament desmoplasty for treatment of intermittent upward fixation of the patella in 24 horses (2005-2012). Can Vet J. 2015;56(2):193-195.
9. Bathe AP, O'Hara LK. A retrospective study of the outcome of medial patellar desmotomy in 49 horses. American Association of Equine Practitioners. December 4, 2004. Accessed June 11, 2026. https://www.ivis.org/library/aaep/aaep-annual-convention-denver-2004/a-retrospective-study-of-outcome-of-medial-patellar-desmotomy-49-horses
10. McIlwraith CW, Nixon AJ, Wright IM, eds. Diagnostic and Surgical Arthroscopy in the Horse. 4th ed. Elsevier Mosby; 2014.
11. Nixon AJ, ed. Equine Fracture Repair. 2nd ed. Wiley-Blackwell; 2019.
12. Baccarin RYA, Martins EAN, Hagen SCF, Silva LC. Patellar instability following experimental medial patellar desmotomy in horses. Vet Comp Orthop Traumatol. 2009;22(1), 27-31. doi:10.3415/vcot-07-11-0100
13. Wright RW, Boyce RH, Michener T, Shyr Y, McCarty EC, Spindler KP. Radiographs are not useful in detecting arthroscopically confirmed mild chondral damage. Clin Orthop Relat Res. 2006;442:245-251. doi:10.1097/01.blo.0000167670.03197.c2
14. Kidd JA, Lu KG, Frazer ML, eds. Atlas of Equine Ultrasonography. Wiley-Blackwell; 2014.
15. Gibson KT, McIlwraith CW, Park RD, Norrdin RW. Production of patellar lesions by medial patellar desmotomy in normal horses. Vet Surg. 1989;18(6):466-471. doi:10.1111/j.1532-950x.1990.tb01128.x
16. McIlwraith CW. Osteochondral fragmentation of the distal aspect of the patella in horses. Equine Vet J. 1990;22(3):157-163.