The term “bone bruise” has come into common use in horses since the advent of medical MRI. Rather than the well-known “bruise”, which comes up following a blow to soft tissues, the term encompasses a variety of types and causes of injury — grouped together due to their similar appearance on MRI.
Bone bruise is one of a number of names given to a specific appearance on MRI. Other terms used to describe the same MRI features include bone marrow lesions, bone oedema, bone contusions and occult fractures. The term bruise implies a traumatic cause, but the distinction relies primarily on a clinical history of trauma.
The characteristic feature of a bone bruise is increased fluid in the bone, most accurately defined by high signal (white) on a specific type of MRI scan — a short tau inversion recovery (STIR) sequence. This scan suppresses the fat signal from bone, which is important because fat, which is normally found in the centre (medulla) of the bone, contains a lot of fluid. If not suppressed, this would hide bone bruises.
The high fluid signal can represent a few different changes in the bone, including bleeding, pericellular oedema, fibrosis or fat necrosis. Because the same appearance on MRI can occur as a consequence of all these different changes in the bone, a variety of different pathological processes can underlie the area of high fluid signal. Trauma is one of these, but other quite common examples include bone degeneration, infection and fracture.
Repetitive strain
When a vet uses the term bone bruise to describe MRI findings, the suggestion is that the injury has a traumatic cause. This can be an acute traumatic episode, or a result of repetitive trauma.
In contrast to what people tend to think of when hearing the term, it is rarely external trauma (such as a knock or blow to the bone) that causes the bruise. Instead, it is more commonly trauma from abnormal and excessive loading of the bone. Examples include landing badly following a jump, or even twisting or slipping while running in a paddock.
Bone bruising from repetitive trauma occurs when a horse undertakes the same training exercises repeatedly. This can affect horses performing in different disciplines, but most often is seen in those training the hardest — for example racehorses and high-level sport horses.
Any horse training to perform well athletically is at risk, no matter how sympathetically he is ridden or on which surface he is exercised. Why one horse will develop a bone bruise while another will cope well with the same training is down to the multifactorial nature of the injury.
External factors such as training load, the make-up of training (the intensity or variety) and surface are all important. Factors individual to the horse will also play a role — conformation, for example, or bone structure, age (or maybe more importantly, “training age”) and genetics.
Bone bruises cause no external swelling and cannot be localised by examination of the leg. The clinical signs are non-specific and could result from many different types of injury.
To make a diagnosis, the vet will first need to localise the source of lameness with nerve blocks and then undertake an MRI scan. An MRI is usually only performed, however, after more conventional imaging techniques such as X-ray or ultrasound. This is because, when compared to MRI, they are more readily available, quicker, cheaper and easier to perform, often enabling diagnosis of the lameness cause.
With acute traumatic injuries, the horse is usually suddenly and severely lame with no warning signs. With repetitive trauma, horses often show transient or intermittent and often slight lameness that becomes more consistent. The only warning signs may be subtle, such as a reluctance to undertake specific manoeuvres or jumping efforts.
Rest or repair?
The mainstay of treatment is rest. Bone generally has a good healing capacity, and will remodel and heal if the abnormal loading stimulus which caused the injury is removed.
There are exceptions. In some cases the extent of the injury to the bone may be irreversible, or have caused structural damage to an adjacent joint.
The type of rest prescribed depends on the cause and the severity of injury. Box rest is most appropriate for severely lame horses following acute trauma. In contrast, some cases with repetitive stress-induced injuries may be best managed with pasture exercise. This allows the bone to “detrain” while receiving a more normal loading stimulus as the horse convalesces in the field.
Additional treatments are often recommended. Pain relief and anti-inflammatory treatment is provided with NSAIDs (non-steroidal anti-inflammatory drugs) such as phenylbutazone or meloxicam.
Aspirin may be prescribed to enhance blood supply to the injured area. Bisphosphonates, such as tiludronate or clodronate, are often used to treat bone pain, but are not appropriate in subchondral bone injuries as they can inhibit the cellular processes necessary for healing to occur.
Occasionally, surgery to place a screw into the bone bruise is recommended — most commonly where the bruise surrounds a short fracture line. Too few cases have been treated in this way to make firm recommendations on when this is best undertaken and the chances of success.
Overall, prognosis for recovery is fair. Much depends, however, on the underlying cause. If an adjacent joint has been damaged, arthritis may develop and this is both progressive and incurable.
Where the injury is a result of repetitive stress, returning the horse to work may cause a recurrence of problems. Because of this, consideration should be given to changing the training regime and make-up when he does resume exercise.
It is impossible to safeguard completely against bone bruises. Repetitive stress cannot be avoided in training for high levels of athletic performance.
Currently, there are no useful tests to detect developing bone bruises. General recommendations to minimise repetitive stress may help, such as implementing a varied work regime on different surfaces. Hacking and hill work, water treadmill exercise and/or swimming can all be used to develop muscular and cardiovascular fitness while minimising repetitive skeletal loading.
Ref Horse & Hound; 30 November 2017
