Strain of the upper part of the suspensory ligament, close to where it attaches to the back of the cannon bone, is a common cause of both forelimb and hindlimb lameness, particularly for competition horses.
Early diagnosis is crucial for a good outcome, because recent injuries are generally less severe and have a better capacity to heal fully. However, early diagnosis is not easy.
It is quite common for there to be no localising signs such as heat, pain or swelling. This is because the ligament lies on the back of the cannon bone, between the two splint bones, and is covered by the check ligament and the flexor tendons.
Diagnosis is dependent on first localising the pain to the area by the use of nerve blocks and ensuring that adjacent structures are not a source of pain. This may mean performing additional nerve blocks, such as blocking the lowest joint of the hock.
Once pain has been localised to the area, several different imaging techniques are used to determine whether it is a bony problem, such as a stress fracture at the back of the cannon bone, a joint problem, such as osteoarthritis of the lowermost joint of the knee, or a soft tissue injury.
Ultrasound is used to identify tendon and ligament injuries, while X-Rays examine the bones and joints.
A specific diagnosis of strain of the top of the suspensory ligament therefore requires definitive localisation of pain to the area and recognition of structural abnormalities of the ligament using diagnostic ultrasound.
These abnormalities can include enlargement of the cross-sectional area of the ligament, poor definition of one or more of the margins of the ligament and some disruption of its internal architecture – seen as a change in the intensity of the ultrasound dot pattern within the ligament.
Although this sounds quite straightforward, it requires considerable expertise and experience – especially in hindlimbs – because overlying blood vessels can create confusing shadows and the tops of the splint bones can get in the way. With good quality images, the severity of the injury can be graded.
Prognosis for recovery is based on the severity of the injury, which is not always directly proportional to the degree of lameness.
In very chronic injuries, the ultrasound images may be misleading because the dot pattern superficially appears normally.
With recent forelimb injuries, lameness can be very transitory and resolve rapidly with rest, only to recur when full work is resumed.
The majority of forelimb injuries respond well to a three-month period of box rest and controlled walking exercise.
The ligament heals and this can be monitored using ultrasound. Most horses return to full athletic function without recurrent injury, so no additional treatment is required.
Early hindlimb injuries may result in low-grade lameness that is easily overlooked until the damage and lameness increases.
In the hindlimb, rest alone is successful. The enlarged ligament and surrounding tight overlying membrane within the small area between the splint bones and overlying soft tissues can create pressure, contributing to pain and lameness. Local nerves may also be compressed.
With very recent hindlimb injuries, injections of anti-inflammatory corticosteroid drugs around the ligament may help reduce swelling and decrease nerve compression.
Chronic or recurrent injuries provide more of a therapeutic challenge. Injuries in the hindlimb are often well-established when first recognised. The prognosis with rest alone is more guarded, with a high proportion of horses remaining lame.
Shock wave or radial pressure wave treatments have been used both for pain management and to stimulate ligament repair.
There is a significant proportion of horses with chronic injuries which remain lame. Stem cell treatment certainly has potential merit, but the results at the moment seem to be completely unpredictable.
In the hindlimb, the suspensory ligament derives its nerve supply almost exclusively from a major branch of the tibial nerve. By surgically removing a piece of this nerve at the hock, it is possible to remove the pain associated with the ligament, but this does not treat the primary injury.
This denervation technique works in the hindlimb only, because in the forelimb the ligament receives innervation from both the ulnar and meridian nerves, and cutting both would not be safe.
Some horses seem to have progressive degenerative injuries, which do not respond well. The neurectomy technique cannot produce a greater degree of improvement than that seen after local anaesthaesia, so it is therefore crucial to analyse critically the nerve block response before selecting a horse for surgical treatment.
Theoretically, a denerved horse should not compete under rules.