What are Rupture of the Fibularis (Penoneus) Tertius?
The fibularis tendon is a part of the reciprocal apparatus of the hind limb, which is a series of tendons that coordinates the simultaneous flexion of the hock and stifle joints. This coordination enables the successful movement of the leg. A healthy fibularis tendon will stretch as the hock joint extends, and serves to help center the weight of the upper part of the limb on the hock. Once damaged, the tendon can no longer do its part and movement becomes compromised. If a laceration is the cause, other important structures of that region can also be damaged, which can complicate healing.
The fibularis (peroneus) tertius is a tendon that originates on the femur and extends across the tibia to the metatarsal region in the hind limbs of horses. A rupture of that tendon can be caused by a laceration or trauma, often due to an overextension of the hock joint. Though an uncommon condition, in severe cases, it can cause lameness. Many cases, however, can be resolved with treatment.
Symptoms of Rupture of the Fibularis (Penoneus) Tertius in Horses
Despite the rupture, pain does not seem to be characteristic of this condition, and affected horses can still bear weight on the injured limb. Symptoms of a fibularis tendon rupture include:
- The hock joint can be extended while the stifle joint is flexed, often seen while walking
- The stifle joint and hock joint can be moved independently from one another
- The lower leg appears to hang limp during walking, as do the muscles in the back of the limb, the superficial digital flexor and the gastrocnemius
- A characteristic dimple can be seen in the soft tissues in the back of the lower limb, or caudal muscles of the crus
- Lameness, most often seen during a trot accompanied by a delayed forward movement
- Swelling near ruptured area
- Wound from a laceration
Causes of Rupture of the Fibularis (Penoneus) Tertius in Horses
The cause of a rupture in the fibularis tendon is a traumatic injury. This can occur from:
- Blunt trauma, such as from falling down, slipping, getting caught in bars, or running into solid objects
- Laceration, most commonly seen from getting caught in a wire
- Hyperextension of the limb, such as from a fast start, or from getting a leg caught on the stable door while trying to jump out of the stable
- Full limb casts that have been applied to hind limbs
Diagnosis of Rupture of the Fibularis (Penoneus) Tertius in Horses
Diagnosis is based on clinical signs, which are characteristic of this particular injury. Manual extension of the hock joint is performed simultaneously with a flexion of the stifle to verify injury, a manipulation that is impossible with an uninjured tendon. The exact location of the rupture can be found through ultrasounds, X-rays, and through exploring a laceration.
Treatment of Rupture of the Fibularis (Penoneus) Tertius in Horses
Treatment follows a conservative course of stall rest for 3 to 4 months, depending on the results of the ultrasound and the severity of the injury. This can include hand-walking. During this time, scar tissue forms between the ends of the ruptured tendon. Function is eventually restored within 2 to 3 months, although it could take up to 12 months for maximum strength to return. After this period of rest, your horse should be turned out to pasture for an average of 12 weeks, and carefully reintroduced to exercise. Your horse should gradually return to his previous level of activity and performance.
Any lacerations or wounds present will also be treated appropriately. Evaluation of your horse’s healing can be made with an ultrasound to prevent a premature return to activity and exercise. A premature return can result in a re-injury of the tendon, which could lead to a severe case of lameness. If your horse does not overcome lameness after treatment, euthanization is often recommended.
Recovery of Rupture of the Fibularis (Penoneus) Tertius in Horses
Follow your veterinarian’s timeline for stall rest and the reintroduction of exercise to ensure your horse heals correctly. A successful return to athletic performance and resolution of symptoms often follows treatment, although performance horses are less likely to return to their intended activity. Factors that influence a successful and quick healing process include the location and degree of the rupture, as well as if there are any other tendon or muscle injuries involved, seen more often in cases of a laceration.