Tendons and Ligaments
Introduction
The main tendons and ligaments usually involved in lameness are the deep and superficial flexor tendons, the check ligament and the suspensory ligament. Tendons attach muscle to bone and are relatively inelastic compared to muscle. The structure of a tendon is fairly complex. There is a surrounding connective tissue sheath which contains many tendon fibres, composed of collagen type 1, that are in a zig-zag pattern which straightens as the tendon is stretched. If a tendon is stretched too far a strain injury occurs. When a tendon heals after an injury collagen type 3 is formed, rather than type 1, which is less elastic causing the structure to be susceptible to re-injury. The superficial digital flexor tendon lies just beneath the skin and attaches to the short pastern bone (second phalanx). The deep digital flexor tendon lies just beneath this and attaches to the pedal bone. Ligaments attach to bone but not muscle and are very similar to tenons in their structure, however, are less elastic. The suspensory ligament attaches at the back of the canon and runs beneath the flexor tendons. Two thirds of the way down the canon the ligament splits in two and attaches onto the sesamoid bones at the back of the fetlock. Branches from here then attach onto the front of the pastern. The check ligament fuses with the deep digital flexor tendon half way down the canon bone and attches onto the back of the knee or hock.
Tendon strain (tendonitis)
There are two flexor tendons running down the back of the limb, the superficial digital flexor tendon (SDFT) and the deep digital flexor tendon (DDFT). As their names suggest the DDFT runs below (deep) to the SDFT. Sprain of the tendons usually occurs during exercise, especially towards the end of a race or event as the horse tires. The SDFT is more commonly damaged than the DDFT. SDFT injuries usually occur in the middle portion of the canon bone, whereas the DDFT is usually damaged lower down, towards the fetlock joint or below.
Signs
Initially the horse will show a sudden, moderate to severe lameness with no swelling. Swelling then develops rapidly often producing a ‘bow’ appearance. In some cases the injury may not be apparent for a day or two. The lameness will usually respond very well to box rest.
Diagnosis
Although the history and examination of the leg are enough to make an initial diagnosis a tendon scan (ultrasound) should be performed to assess the extent of the injury and, therefore, provide better information on treatment and likely outcome. Tendon scanning is often delayed until a few days after the injury occurred as the damaged tissue can be seen more easily and the extent of the injury more fully assessed. Ultrasound is also used regularly during treatment to assess healing of the tendon.
Treatment
Initially the horse should be placed on box rest and regular cold water hosing used to help reduce the swelling. A thick bandage can be applied to the affected leg and anti-inflammatory drugs, such as phenylbutazone, (‘bute’) used. During this acute stage fibrous tissue heals the damaged tendon. Once this initial, acute, phase has finished the horse must continue rest. The rest here is called active box-rest as the horse needs to move around to help turn the fibrous tissue in the tendon to more normal elastic tissue (collagen). The final stage is a controlled exercise program to bring the horse back into work. As previously mentioned ultrasound scanning should be used to assess healing throughout the healing process due to the complexity of the healing. The total time for horses to be out of work can be up to 18 months, the longer the time off the better to help prevent recurrence of the injury. There are other treatments available to help maximize tendon repair, for example the injection of stem cells into the tendon, however, these are not used routinely in equine practice.
Tendon rupture
Rupture of the superficial digital flexor tendon is basically a severe form of strain where full thickness damage to the tendon has occurred. The fetlock joint of affected horses will ‘drop’ when the other leg is lifted and the horse will show a severe lameness.
Diagnosis
Although the history and examination of the leg are enough to make an initial diagnosis a tendon scan (ultrasound) should be performed.
Tendon laceration
Flexor tendons
This usually occurs from a sharp wound to the back of the leg. The superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT) and suspensory ligament (SL) can be involved. If just the SDFT is involved the fetlock joint horses will ‘drop’ when the other leg is lifted, if the DDFT is involved the horse’s toe will lift off the ground, if all three structures are lacerated the fetlock joint will rest on the ground. In some cases the digital sheath may be damaged, see section on digital sheath.
Diagnosis
Scanning (ultrasound) is usually used to confirm a diagnosis and assess which structures are involved.
Treatment
Ideally the tendons should be stitched back together surgically. This usually requires extensive cleansing and removal of dirty or damaged tissue (debridement). The affected leg should be placed in a cast for 6-12 weeks. In some cases the tendons cannot be sutured and, therefore, treatment is prolonged rest with immobilization (cast/bandage). The prognosis for many lacerations is unfavourable for a return to work but this largely depends on the extent of the damage.
Extensor tendons
Laceration of the common (CDET) or long digital extensor tendon (LDET), running down the front of the canon, will produce either no abnormal gait whatsoever or ‘knuckling’ of the fetlock.
Treatment
Surgery is not usually required for these animals as the tendon will usually repair with a period of rest. If there is excessive ‘knuckling’ the leg can be bandaged, although this is not usually necessary.
Suspensory ligament desmitis (strain)
The suspensory ligament runs from the top of the canon, attaches onto the sesamoid bones at the back of the fetlock and then runs down the side of the fetlock to join the extensor tendons and is deep to the flexor tendons. Strain of the suspensory ligament can occur at three different points, the attachment of the ligament at the top of the canon (‘high suspensory’), the body of the ligament (as it runs down the back of the canon) or a branch injury (the ligament splits in two before it attaches to the sesamoid bones). The most common injury to the suspensory ligament is strain of one or both branches.
Diagnosis
Ultrasound scanning is used for diagnosis and to assess treatment options. In many cases, where the lameness is slight and has been there for a long while (chronic), nerve blocks are required to localize the lameness to the suspensory ligament.
Treatment
The horse should be placed on box rest and regular cold water hosing used to help reduce the swelling. A thick bandage can be applied to the affected leg and anti-inflammatory drugs, such as phenylbutazone, (‘bute’) used.
The length of time of box rest depends on how the injury is healing, using ultrasound scanning, and can vary from 4 weeks to 9 months. Horses must then start a controlled exercise program to bring them back into work.
Check ligament desmitis (strain)
The inferior check ligament is a small structure that connects the deep digital flexor tendon (DDFT) with the back of the canon bone. Affected horses will usually have heat and swelling in the region towards to the top of the canon bone and have a variable lameness. In some instances there is also damage to the deep digital flexor tendon (SDFT).
Diagnosis
Ultrasound scanning is used for diagnosis and to assess treatment options.
Treatment
The horse should be placed on box rest and regular cold water hosing used to help reduce the swelling. A thick bandage can be applied to the affected leg and anti-inflammatory drugs, such as phenylbutazone, (‘bute’) used. The length of time of box rest depends on how the injury is healing, using ultrasound scanning. Once the vet deems it appropriate horses must then start a controlled exercise program to bring them back into work. The time it takes to get the horse back into work can vary from between 3 to 9 months.