Disordered Physiology and Hoof Morphology - The Chicken and the Egg
The physiological effect of poor conformation and hoof morphology on the wider skeletal system of the horse is a highly complexed subject, as touched on by Simon Curtis (2002) the variations of both are limitless and therefore their physiological effects differ vastly, however we can apply the laws of physics to predict the predispositions of certain conformations.
Poor hoof conformation effects the musculoskeletal system and hoof morphologies are caused by conformation defects and both can be perpetuated by the lack of recognition, a limb with a deviation will cause the foot to distort over time, leading to the damaging of the internal structures, so we have a chicken and egg situation when it comes to whether hoof morphology influences the wider physiology or vice versa.
Even with an ideal conformation, hoof morphology will have a wider effect on the skeletal system, there are many papers that study the physiological effect of hoof morphology on the internal structures of the digit, Van Heel et al (2004) and Van Heel et al (2005) along with Moleman et al (2006) all describe how the centre of pressure (CoP) of the hoof moves caudally as the hoof grows putting more pressure on the navicular region and the deep digital flexor tendon. Wilson et al (1998) talks about medio lateral and dorso-palmer imbalances of the hoof on the wider skeletal system, he discussed how the CoP moved towards the higher side of the imbalance, this in turn puts uneven loading on structures higher up, predisposing to injury, these effects are exacerbated in poorly conformed horses.
Poor medio lateral hoof morphology effects the wider skeletal system by causing uneven loading of the limb, all structures on the high side suffer from compressive forces and the low side from strain.
Fig 1 shows side bone, the lateral cartilages are one of the structures effected by medio lateral imbalance, when the hoof lands, the high side will make contact first, taking most of the impact forces and then subsequently as shown in Wilson et al (1998) will continue to bear more load during the stride. This uneven loading will predispose to damage of ligaments and joints all the way up the limb, an example being splints (uneven loading of the metacarpal bones causing damage to the interosseous ligaments). Often the articular joints of the limb can be almost touching on the compressed side, when loaded this can cause damage to the articular cartilage as the bones collide, in some cases causing OCD as a chip of cartilage and/or bone is broken off and floats within the joint capsule, or the cartilage can suffer excessive wear on the load bearing side. Medio lateral imbalance will also cause the animal to change its stance to try and counter the imbalance, this can predispose the animal to muscular issues and changes in muscular development (Kilmartin 2014).
Poor mediolateral morphology can be caused by farriery (repeatedly trimming imbalanced) or by conformation, as discussed by Simon Curtis (1999) angular limb deformities are a route cause of poor medio lateral hoof morphology. He discusses how the horses hoof will morph according to the loads and stresses it is exposed to, this is expanded on in fig2 and Fig3. If the horse is base wide and has knock knees (carpal Valgus), it will have a hoof capsule that is distorted laterally so that it flares on the lateral wall, is upright or under-run medially and may have the medial bulb shunted proximally. Its action will dish, and it will probably land on the lateral hoof wall, smack down hard on the medial side during the weight bearing phase and breakover on the inside toe. If its uneven landing is severe it will be prone to corns and even quarter cracks. (Curtis 1999 Foal to race horse pg. 3)
Poor mediolateral imbalance can also predispose the horse to interference injury as the conformational defect effects limb flight. Fetlock Varus for example, as discussed by Curtis (1999) may cause the hoof to be upright laterally, flared medially and cause an axial movement in flight causing interference injuries with the opposing limb.
In the ideal conformation the loads and strains of the horse are evenly distributed throughout the hoof, this allows for a balanced hoof morphology as the forces acting upon the hoof are what is responsible for the hoof shape fig2.
Fig 3 helps to explain how poor hoof morphology is perpetuated by poor conformation and other environmental influences, as discussed by Curtis (2002) form follows function, this poor morphology is likely caused by poor conformation and then the cycle is perpetuated due to the mechanical function of the hoof being compromised.
The effect that distal limb deformities can have on the loading forces imposed on the hoof capsule is demonstrated in fig.4. the blue line indicating the direction of the centre of force descending the limb. As stated by T. Stashak (2002) base narrow conformation causes the horse to bear more weight on the outside of the foot and a base wide horse causes the foot to bear more weight medially, these imbalances will inherently cause poor hoof morphology.
Poor medio lateral balance can also give rise to sheared heels as discussed by T. Stashak (2002) as one heel is repeatedly subjected to more load. Sheared heels can then lead to other predispositions such as chronic heel soreness, hoof cracks in the bars or quarters and deep thrush in the central sulcus as the frog essentially splits in half.
Poor dorso-palmer hoof balance (DPHB) is a major cause of lameness in sports horses, Witte (2014) discussed how DPHB has a direct effect on the centre of pressure (CoP) within the hoof and the lever arms of the toe and deep digital flexor tendon (DDFT), a horse with long toes and low heels or a broken back hoof pastern axis (BBHPA) will have a long lever between the point of rotation and the tip of the toe, this requires an increase force from the DDFT to initiate breakover, this subjects the DDFT and the whole navicular area to increased load and strain predisposing them to injury. T. Witte statements are backed up by Van Heel et al (2004) Van Heel et al (2005) and Moleman et at (2006) who all discuss the movement of the CoP in relation to hoof morphology.