DEEP BRUISING and soft tissue damage are just two of the terms that have long been used to describe sources of foot pain that elude traditional diagnostic techniques. Such horses would typically present with lameness that could be localised to the hoof by nerve blocks but there would be no abnormalities on X-ray images of the foot.

DEEP BRUISING and soft tissue damage are just two of the terms that have long been used to describe sources of foot pain that elude traditional diagnostic techniques. Such horses would typically present with lameness that could be localised to the hoof by nerve blocks but there would be no abnormalities on X-ray images of the foot.

The trouble with X-rays is that they do not identify damage to the tendons and ligaments inside the hoof and, if there is bony involvement, this may not be apparent in the early stages of an injury. Hence the assumption that, in the absence of radiographic changes, there was an injury to the non-bony or soft tissues of the foot.

Ultrasound, which is commonly employed to diagnose tendon and ligament sprains elsewhere in the horse, is little use in these cases because sound waves do not penetrate the hoof wall. Scintigraphy (bone scan) is also of limited benefit because it does not adequately localise the site of injury and gives no information on the nature of any damage.

A horse undergoing a standing MRI scan, with its off-hind foot placed inside the horseshoe-shaped magnet of the scanner

Magnetic resonance imaging (MRI) overcomes these problems by providing exquisitely sensitive images of both the bone and soft tissues inside the foot. It identifies the location of the injury and determines its extent, which are key factors in deciding upon the best treatment and determining the outlook for the horse.

The technology behind MRI involves placing the part of the body to be imaged inside a strong magnetic field.This bombards the body with pulses of radio waves, which are not harmful, and caused the tissues to return a signal that is detected by a receiver.

The body is scanned slowly and the signals emitted are a property of the tissue type - be it bone, ligament, tendon, blood or inflammatory fluids. Computer-capture of these signals allows a detailed image to be built up in either 2-D or 3-D, providing an unparalleled view inside the body.

Two types of MRI scanners have been developed for horses. The first machines were adapted from human MRI systems and required that the horse be anaesthetised for the leg to be placed in the scanner. More recently, a dedicated system that allows the horse to stand inside the MRI scanner has been produced.

A standing MRI scan is performed with the horse sedated and the shoes removed. The procedure takes between one and three hours depending upon the horse's temperament and the area being scanned; MRI images can be obtained of the fetlock and cannon bone regions as well as the foot. This system is in use at the equine hospital of the Dick Vet in Edinburgh, as well as several specialist equine practices in England.

MRI scans are indicated for horses with long-standing lameness of one or more feet with no significant changes on X-rays. These horses will already have undergone nerve blocks to localise the pain to the foot, together with specific nerve blocking of the coffin joint and, or, navicular bursa.

A range of bony and soft tissue abnormalities are identified by MRI, and it is not uncommon for there to be damage to several structures inside the foot. In a recent survey, the most frequent injuries were to the deep digital flexor tendon and the navicular bone, which were present in approximately 30% of cases.

Importantly, irregularity of the navicular bone was often identified despite there being no obvious findings on X-rays, which is the conventional way of examining this part of the foot.

Damage to the navicular bursa was present in about 15% of the horses. Injuries to the ligaments around the coffin joint were less common.

Having established the source of lameness using MRI, the findings are then used to provide the right treatment in the form of medical therapy, corrective shoeing and rehabilitation. This is a challenge.

Cynics might argue that, for all the diagnostic brilliance of MRI, it does little else other than to simply confirm that the horse is either stuffed' or needs a long period of box rest.

I would beg to differ. Yes, the prognosis for those horses that have damage to their deep flexor tendons is poor, even with six or more months of stable confinement, but knowing the location of the injury does offer the possibility of injecting stem cells or other potential remedies.

Also, there is much that we can do for the other types of injury identified by MRI. Coffin joints and navicular bursae can be injected with anti-inflammatories to provide direct and concentrated medication, significantly improving the outlook.

Collateral ligament injuries do heal well in my experience, with affected horses returning to full competition soundness. Remedial shoeing also becomes much more effective once the source of the problem is known.

The development of MRI has undoubtedly revolutionised understanding of injuries that occur inside the hoof and is helping their management, but this does come at a price.

The equipment is not cheap to purchase, install and maintain. Owners should expect to pay between £900 and £1200 to have their horse undergo an MRI scan.

You should also check whether the procedure is covered by any insurance, since some policies exclude or provide only a contribution to the cost of an MRI.