Inside the stifle: explaining injuries in horses within this joint *H&H Plus*

How significant are soft tissue injuries within this complex joint? Matt Smith MRCVS explains

THE stifle is a hard-working joint with a large range of motion, which connects the horse’s thigh and crus (lower limb) regions.

It actually comprises two joints. The femoropatellar joint is located at the front of the stifle, between the ridges at the bottom of the femur and the patellar (the equivalent of our kneecap). The femorotibial joint sits between the bottom of the femur and the top of the tibia, and is divided into medial (inside) and lateral (outside) compartments.

Unlike in most joints, where the opposing cartilage surfaces fit into each other to create a smooth, gliding surface, the femorotibial joint features smooth discs of tissue called menisci, which sit between the ball-shaped ends of the femur and the plateau at the top of the tibia.

A complex array of ligaments supports the stifle and provides stability. On the inside and outside are the collateral ligaments. The patellar ligaments connect the patellar to the tibia, at the front, forming a functional extension of the quadricep muscles. In the centre, the criss-crossed cruciate ligaments connect the bottom of the femur and the top of the tibia.

Tissue injuries

STIFLE injuries are quite common and the soft tissue structures are frequently involved. The range of issues is similar to those seen in the human knee, with meniscal injuries occurring more often than damage to the cruciate and collateral ligaments.

Injury usually follows trauma to the joint. Lameness, often severe, is usually the first clue. While this can improve with rest, moderate lameness typically persists.

Fluid swelling of the stifle joints will be apparent when the lame leg is examined. In the most severe injuries, there may be instability of the joint and obvious swelling around the stifle as a result of internal bleeding. Localising the cause of lameness to the stifle may be possible from examination, but nerve blocks may be necessary.

Ultrasound is generally more useful than X-ray for further investigations, but there are pros and cons with each technique. Fragments of bone that have pulled away from the attachment sites of the menisci and stifle ligaments can be seen with X-ray, as can other abnormalities such as reactive changes within the joints.

While ultrasound is very effective for imaging the menisci and collateral ligaments, significant portions of the menisci between the joint surfaces are not visible. Views of the cruciate ligaments are also limited.

More advanced imaging techniques may be needed to establish a diagnosis. Arthroscopy (keyhole surgery) is often used, as this allows direct examination of the interior of the joints and offers the best treatment in many cases.

Experience with computed tomography (CT) and magnetic resonance imaging (MRI) is developing in the investigation of stifle injuries, as both allow thorough evaluation of the bone and soft tissues. It is likely that these techniques will become more commonly used, but they add significant expense.

Treatment options

MUCH of what we know about soft tissue injuries in the stifle has come from observations during diagnostic investigation of lame horses using arthroscopy.

Injuries of the meniscus most frequently occur within the medial femorotibial joint. In people, the entirety of the upper surface of the menisci can be evaluated with arthroscopy, by applying traction to the knee and pulling the joint surfaces apart to give space to look around with the arthroscope. This is not possible in the horse, however, meaning that portions of the meniscus, particularly towards the centre of the joint, cannot be seen. Injuries in this location are also inaccessible for arthroscopic treatment.

In the majority of cases, arthroscopic surgery involves removal, or “debridement”, of torn and disrupted portions of soft tissue. Partial meniscectomy (removal of the meniscus) can be performed or, with cruciate ligament injuries, torn ligament fibres removed.

During arthroscopic surgery, the cartilage surfaces are also evaluated for any additional damage or signs of degenerative (arthritic) change. The information obtained provides the best guide for the horse’s chances of recovery from injury.

Intra-articular medications may be used as an adjunct to arthroscopic treatment, or, with mild injuries, as the primary treatment. Anti-inflammatory injections are commonly used, typically corticosteroids such as triamcinolone (similar to cortisone, often used in people).

Your vet may recommend alternative intra-articular medications; examples include hyaluronic acid, which is both anti-inflammatory and viscoelastic (shock absorbing), or polyacrylamide gel, which is thought to form a cushion-like membrane after injection. Stem cells have also been researched for treatment of meniscal injuries, and may improve healing.

Following treatment, an effective rehabilitation programme will help the horse back into work.

Meniscal tears most frequently occur in the medial femorotibial joint. The left scan shows a tear before treatment, and the right following debridement (removal) of torn tissue

The prognosis

MANY horses can make a return to full competitive soundness following arthroscopic surgical treatment. The prognosis depends upon the extent of the injury. With meniscal injuries, approximately two-thirds of horses with minor tears recover fully. This falls to closer to 50% with moderately severe tears; in the worst injuries, very few can resume work.

Injuries of the cranial cruciate ligament treated by arthroscopic surgical debridement generally have a fair prognosis, with approximately half of horses reported with this injury resuming a competitive career.

Sadly, horses who sustain a complete rupture of either of the cruciate ligaments are unable to work again. In the most severe soft tissue injuries, stability of the joint may never completely return and arthritis is likely to develop, with associated persistent lameness.

The vet

MATT SMITH MRCVS is a surgeon and partner at Newmarket Equine Hospital (NEH). This referral hospital is one of the largest in Europe, treating a wide range of leisure and sport horses from all over the UK. Matt is a Royal College of Veterinary Surgeons (RCVS) specialist in equine surgery and has a particular interest in orthopaedic problems. 01638 782020, newmarketequinehospital.com

This feature is also available to read in this Thursday’s H&H magazine (1 April, 2021)

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