Summary:
Reasons for performing study: External landmarks that are said to relate to internal anatomical points are widely used in the farriery profession during trimming but have never been tested for their scientific validity.
Objective: To see how closely external ‘landmarks’ correspond to the internal anatomical structures that they are said to relate to and to test Duckett’s (1990) theory that the Dorsal Wall Length is equal to the length from the distal toe to the centre of rotation of the distal interphalangeal joint.
Methods: Twenty three cadaver limbs were removed above the carpus to preserve suspensory attachment and trimmed to a set protocol. They were then loaded to mid-stance in a leg press. Radiographs were then taken and images loaded onto Ontrack™ for subsequent measurement and analysis.
Results: From the measurements taken in this experiment, the external reference point ‘Duckett’s dot’ should be moved caudally by 4.5mm from the currently recommended position to more accurately reflect its internal anatomical structures.
The dorsal wall height was found to be equal to the distance from the distal toe to the centre of rotation of the distal interphalangeal joint within ± 1mm across the data set. However this was less reliable when applied to the external ‘landmark’ of the bridge due to migration of the ‘landmarks’.
Conclusion: Duckett’s theories have been found on the whole, to be correct. However they are less reliable the further the foot conformation is away from ‘normal’.
Introduction
Foot balance is arguably the most controversial subject in farriery. There is little scientific data on the subject or universal agreement amongst hoof care professionals about it. It is widely recognised that proper trimming and shoeing have an enormous influence on the soundness of the horse (O’Grady and Poupard, 2001). Many veterinary surgeons and farriers assert that a large proportion of lameness seen today could be prevented or treated through good farriery and correct foot balance (O’Grady and Poupard, 2003). However defining and achieving “good farriery” or correct static balance is an area where there is little universal agreement between fellow professionals.
There are numerous different and conflicting theories and practices concerning hoof trimming and balance. Ovnicek (1997) used the natural balance trim citing the frog as being ⅔rd the basal length of the bearing border with a 60:40 ratio of heel to the Centre of Rotation (COR) to break over. Duckett (1990) suggested trimming the feet to the centre of articulation of the distal interphalangeal joint using external reference points. He suggested using “Duckett’s dot & Duckett’s bridge”. According to Duckett, the dot is a consistent external reference point situated 3/8” behind the apex of a trimmed frog and is proportionate from the Shire to the Shetland according to Duckett (2008). The internal anatomical structure the dot represents is the centre of pressure (COP) (Clayton 2001) but is probably more accurately described as the centre of mass (COM) off the foot (Fig1). This is the point where the Common Digital Extensor Tendon (CDET) and the Deep Digital Flexor Tendon (DDFT) inserts on the Distal Phalanx.
What Duckett calls the bridge is thought to represent the centre of articulation/ rotation (COR) of the distal interphalangeal joint (DIPJ) (Fig 1) and is said to be representative of the widest part of the foot. Duckettplaces this at between ¾ to 1 inch caudal to the dot although he is not clear on how he calculated this pointor if this differs from the position of COM.
Figure 1Showing the internal & external relationship between Duckett’s ‘landmarks’ and their corresponding internal structures.
Duckett’s external landmarks represent internal anatomical structures that are in fact important bio-mechanical structures that must be taken into consideration when trimming the equine to achieve effective dynamic foot balance. Dynamic foot balance is achieved when the moment arms affecting the foot at mid-stance are in equilibrium. At this point the opposing forces acting on the foot are neutralized and the foot is not unduly stressed. Without consideration of these moment arms and their corresponding structures Duckett’s dot becomes just an arbitrary measurement. The very essence of Duckett’s theories are the relationship between his ‘landmarks’ and their corresponding structures.