Equine Conformation – the key to soundness

As vets, we most often scrutinise conformation at pre-purchase examinations and as we start lameness investigations.  We also examine conformation for some owners prior to breeding the mare or stallion, in order to minimise the chance of passing on hereditary conformational faults.  By carefully examining conformation, we may be able to identify potential sources of future lameness.

Conformation is a very subjective area and unfortunately there are very few objective measures to determine if a horse’s conformation is ‘good’ or ‘bad’.  Some vets have tried using radiographic, photographic and video analyses to quantify conformation.  As there are so many variables including breed variations, foal and youngstock management and multiple conformational abnormalities identified, it is very hard to pinpoint every aspect and the effect it will have on future athletic performance.  Conformation also does not account for factors such as horse attitude, management, working practices and concurrent health status of that individual – there are plenty of examples of the imperfect horse competing and staying sound at the highest levels of competition.

Conformational assessment

Conformation needs to be assessed from all views on a firm level surface, with the horse standing square as subtle alterations in the ground surface can easily make a limb look like it is deviated even if it is straight.  We also assess a horse’s conformation when it is walking, again from all views.  There are 4 basic principles that can be used to fully assess conformation – balance or proportions; lengths and angles; musculature and limb conformation.

The balance is said to be good if, when the horse is seen from the side, it’s body can be fitted into a square box.  Other people use the rule of 3 interlocking circles visualised on the side of the horse’s body and if they overlap by 1/3 each then the horse should be in balance.  The chest should have adequate width for heart and lungs, and the pelvis should have appropriate width for muscling although these factors are often breed dependant.

The body and limb angles are also important to assess.  Shoulder angles, pastern angles and croup angles are easily seen from the side.  The hind feet should have a slightly steeper angle than the front feet.  The long bones should appear in proportion to the size of horse, with a longer forearm than cannon bone.  The withers should be equal in height or marginally lower than the croup height.

The muscling of the horse, whilst not to do with the horse’s bone structure, is very important, especially when the horse is presented for lameness.  Asymmetric muscling can be a clue that the horse has not been using himself equally and a mild lameness may be present.  Often shoulder, back and hindquarter muscling is best viewed from behind the horse, both at ground level and from above the horse (although this must be done safely!)

The horse shows ‘bench’ or ‘offset’ knees.  The line through the middle of the forearm should continue through the middle of the cannon and foot.

Viewing limb conformation from all angles is the final step in a complete examination of the horse’s conformation.  Using imaginary plumb lines can be a very useful tool to help identify any deviations from a normal straight limb.  From the front, a straight line should pass through the middle of each of the joints, ending in the middle of the hoof.  From the side, the line should fall though the centre of the long bones, the knee and the fetlock and end about 5cm behind the heel of the front foot.  A line from the point of the buttocks should skim the point of the hock, run along the caudal metatarsal area and end about 7-10 cm behind the heel of the back foot.  From behind, the line should run through the midline of all the joints and end between the heel bulbs.  The joints should be flat and without any obvious swellings and the feet symmetrical.

Heritability

It is vital to consider any heritable conformational problems before breeding from the horse.  It appears that the dam contributes more to conformation than the sire, although this may be influenced by stringent stallion licensing or grading which eliminates poor conformation before breeding.  Breeding horses of good conformation is more likely to result in well conformed progeny.

Foot conformation was shown to be moderately heritable and is probably most often thought about as horse’s with ‘typical Thoroughbred feet’.  Other conditions that are viewed as heritable include back at the knee, tied in below the knee, toe in and toe out, offset knees and sickle or straight hocks.  Unfortunately different studies and different breeds seem to show different heritability for the same conditions which means providing definitive answers is difficult.

Some heritable conditions will improve as a foal matures and some cases can be dramatically improved with correct management and veterinary treatment so poorly conformed foals can grow into more correct adults.  This is evident when yearlings are re-examined as 3 year olds; back at the knee conformation improves with age.

Examples of conformational problems implicated in lameness

Toe out conformation. The right limb is worst.

Different breeds and different individuals obviously have different performance requirements but the conformation must still be viewed in light of future soundness of the horse or pony.  This means one conformational fault may stop a horse becoming an elite athlete, but it will still be able to successfully compete at low levels, or pursuing a different career.

Toe out conformation is very commonly seen in warmbloods

As foals mature, mild conformational faults may improve, especially back at the knee conformation and limbs that deviate to the side from the knee downwards.  Foals will also often start life with a broken forward hoof pastern axis, which will straighten out as they mature.  By examining the foal early, advice can be given regarding whether they are likely to improve with time or whether veterinary and farriery treatment would help to improve future conformation.

Horses with flatter feet and lower sole angles are more likely injure their navicular area or deep digital flexor tendons (DDFT), whilst those with more upright foot conformation may be more likely to sustain suspensory ligament injuries.  Again, these faults can be dramatically improved by correct and corrective farriery throughout the horse’s life.  Indeed, part of the management plan after lameness is to address any foot imbalance that is present.

‘Sickle’ hocks are seen with too acute an angle at the hock, meaning the cannon does not sit in an upright position.  This can result in hock associated with lameness.  Equally upright hock conformation can result in proximal suspensory ligament disease

Horses that are back at the knee may also have slightly upright pastern angles.  Carpal hyperextension may increase the likelihood of horses developing fractures of the small knee bones when raced at speed although a small study of thoroughbreds did not support this.  Over at the knee is more often viewed as an acquired conformational fault which means the horse was not born with this, but it has developed.  Often this will become more obvious with age and may even be more pronounced after extreme exercise.

Hocks are deemed to be sickle hocked if the hock angle is very acute and the cannon bones are angled forward under the horse rather than upright.  Interestingly, this problem is virtually not identified in elite performance horses.  Much more commonly, we see straight hocked conformation and relate it to hindlimb lameness, predominantly of the hock area.  We continue to see cases of hock osteoarthritis (bone spavin) and are also seeing an increase in cases of proximal suspensory ligament desmitis which also often related to straight hock conformation.  One team of researchers suggested that a straight hock conformation will absorb less concussion than a sickle hock.

Summary

It is important to take time to identify conformational faults, not just as a veterinary practitioner, but also as a prospective breeder or judge of horses although despite poor conformation some horses still confound us by becoming elite athletes.  Unfortunately evidence based medicine studies are few and far between and therefore objective analysis of conformation and its true effect on future athletic ability remains unclear.

Article provided by Ashbrook Equine Hospital