Introduction to the mechanics of the lower limb and evaluation radiographically and clinically

Introduction to the mechanics of the lower limb and evaluation radiographically and clinically

Sammy L. Pittman,DVM

Innovative Equine Podiatry and Veterinary Services, Pllc

            Considering a large component of lameness occurs in the lower limb and the equine hoof a thorough understanding of the forces at play are very helpful.  We often examine and treat lameness from a medical standpoint but are not fully recognizing and changing the biomechanical properties that are very likely involved in creating the lameness. 

            The detailed anatomy is covered at length in many text, conversely, I want to focus on the functional anatomy as it relates to the mechanical properties of the equine digit.  Consider the deep digital flexor tendon arising from the combined flexor muscle bellies coursing distally over the palmar/plantar aspect of the fetlock and pastern then over the navicular bone to attach to the semi-lunar crest  on the solar aspect of the coffin bone.   The tendon attaches firmly to the bone and the bone is attached to the hoof wall via the lamellar network.  Think of these combined anatomical structures as creating a sling or hammock for the boney column.  See figure 1 for a drawing emphasizing the suspension and support components.  Also consider the frog, ungual cartilages and digital cushion as support structures accepting load  that is determined by the balance of load from the suspension system. 

            To further define the deep digital flexor tendon suspension theory, consider a deep flexor contracture case versus a tendon laxity case in young foals.  The contracture case has no load on the heels as they are suspended in the air via the shortened tendon unit.  Compare to the tendon laxity case in which the toe is popping up and the heels and bulbs are the weight bearing component.   This is a high suspension versus low suspension comparison and further describes how the deep flexor tendon has a great influence on what structures are loaded within the hoof capsule. 

                                                                         

         

 Figure 1 Suspension components and support  components

            Now let's think about what load does to the hoof.  For example compress one side of your fingernail and watch it turn pale in color.  This is a load induced vascular compression that prevents the vascular network from filling.  The same goes for the equine digit.  When weight is placed on the limb the vascular network is loaded and blood moves out of the loaded areas to unloaded areas.  This is easily confirmed by performing venograms.  As long as the compression is temporary and balanced throughout the hoof it is of no consequence.  However when long term compression occurs, bone and soft tissue suffer the effects of decreased nutrient flow.  This is evidenced by lack of growth of sole and/or hoof wall and boney remodeling of the coffin bone.  Consider a high grade club foot versus a crushed heel foot.  Club feet have trouble growing sole directly under the apex of the coffin bone and dorsal hoof wall.  Hooves with tendencies to have long toes and low heels with difficulty growing heel.  These are both load induced vascular compressions secondary the loads determined by the deep flexor tendon suspension. Figure 2 compares a foot with a severe negative palmar angle on the left to a grade 3 club on the right.  The foot on the left has vascular compression under the wings of the coffin bone and the foot on the right has compression under the apex of the coffin bone.  The tighter suspension unit of the club syndrome transmits a greater proportion of the load to the toe. The crushed heel with less deep flexor suspension allowing more load at the heels. 

Figure 2 Negative palmar angle venogram on the left compared to a grade 3 club foot venogram on right.

            Radiographic investigation with properly taken podiatry style radiographs will allow definition of the areas that are chronically loaded. Coffin bone shapes tell us the history of the loads that have been applied to it.  Wolfe's law describes that bone remodels along lines of tension and compression   Coffin bones shapes of club feet have a characteristic bump about halfway down the face of the coffin bone, lipping at the apex and resorption directly under the apex, secondary to forces acting upon these regions from shortened musclotendinous unit.  The articulation will also develop with more dorsal orientation.  Compare to the low heel foot which will have a straight face and tip of the coffin bone with  a scallop resorbed in the wings from the load placed in this region.  The articulation develops further palmarly closer to the wings.  Evaluation of the center of rotation of the coffin joint will show that the more upright clubby type foot has much less coffin bone dorsally when compared to a lower heeled, long toe foot.  This effects the lever arm working against the deep flexor tendon that is necessary to consider when treating the long toe low heel horse. 

Taking consistent radiographs before and after shoeing on all my foot lameness cases allows a greater understanding of the mechanical properties that matter to the horse.  Below is a review of the soft tissue parameters that  I routinely monitor.   

Figure 3 Soft tissue parameters

            Coronary band to extensor process (CE) is measured from top of paste which is applied at most proximal aspect hoof wall at the point of the last hair follicle down to the extensor process of the coffin bone.  This will range from 8 to 30 mm in most healthy hooves.  This number does not give you much information as a single measured parameter.  However, when monitored and compared in serial radiographs, especially when monitoring an acute laminitis case, it is extremely valuable. For example, an acutely laminitic patient that measures 8mm on day 1 of clinical signs and then measures 18mm on day 4.  This is a 10mm  distal displacement which is usually accompanied by a 10mm decrease in sole depth as well and varying degrees of rotational displacement.  

            Horn-Lamellar zone (HL) is measured in two areas, one proximal just below extensor process and one distal just above apex of coffin bone.  This will most commonly measure 15 mm in most light breed horses but can be as high as 20mm in larger breeds, mules and donkeys.  This measure is expressed as proximal HL/Distal HL (15/15).  Instead of measuring only rotation this will give you a measurable displacement that is more definitive than a generic rotation.  Evaluating the dermal-epidermal junction is also of great importance as it should split the horn lamellar zone further defining each.  This allows more specific interpretation of changes in the HL zone.  For example with laminitis the L component of the HL zone will change not the H component.  Early in laminitis this may be the only notable change and an increase of 3-4 mm is a significant finding and may have no measurable rotation.  Several important disease processes can be discovered in this zone and many foot diseases such as clubs, chronic/acute laminitis, white line disease, keratomas and abscesses have very unique qualities that can be shown here.

            Sole Depth (SD) is measured from the tip of the coffin bone down to most distal aspect of the sole.  The cup is also of importance as it is present to different degrees depending on health or pathology and can also be falsely created with a hoof knife.  This measurement is expressed as SD/Cup.  Healthy feet with no pathology will most commonly carry 15mm of sole and a 2-3mm cup (15/3). This should be of upmost concern of the vet/farrier team when striving to obtain soundness and health of the foot.  This should be the measurement at the day of the farrier visit.  Often thin soled horses are at 6-7mm of sole 8 weeks into the cycle and this is a sign of a compromised foot that requires a different approach to increase foot mass and health. Two measurements can be made to give you more information, one at tip of coffin bone and one under wing of coffin bone.  Venogram findings suggest that a depth of 15mm is required to maintain a healthy appearance to the solar vascular bed with robust and correctly aligned terminal papillae

            Digital Breakover (DB) is measured from the tip of the coffin bone to where the foot or shoe if shod would leave the ground.  Healthy hooves that maintain adequate SD and good digital alignment will commonly maintain a DB of 20-25mm.  Many times in perimeter fit shoes, depending on type of foot, bone angle, and toe lever this number is considerably higher than ideal at the day of the farrier visit and continues to lengthen throughout the cycle due to hoof growth. This gives us a measurable lever arm that applies its force to the deep digital flexor tendon and its subsequent force impacts on apex of the coffin bone, dorsal hoof wall and navicular apparatus.  Below I discuss toe lever (TL) that in my opinion gives a more accurate understanding of the lever arm involved. 

                Toe Lever (TL) can be expressed as static toe lever or shod toe lever.  Shod TL is  measured from center of rotation of the coffin joint to where the hoof/shoe would leave the ground and static TL is measured from the center of rotation to the tip of the coffin bone.  Shod TL we can effect and static we cannot.    Lower BA coffin bones typically  have a longer TL than higher degree.  In my practice I see static TL as short as 45mm to as long as 75mm in adult horses.  Monitoring this at a young age may allow us to apply orthotics that will decrease the effective lever arm that antagonizes the lower limb.  Therapeutic shoe packages can be evaluated with regard to amount of lever arm relief.  Simply setting the shoe back only effects this measurement a few millimeters and sometimes many lameness issues respond to a TL that is 3-4 times less than what is measured on their bare foot. 

            Bone Angle (BA)  is the angle of the coffin bone when viewed in a lateral radiograph.  Average BA will be 50 degrees.  In my practice I have measured BA's as low as 36 degrees in very low heeled and long toed horses to 70 degrees in club feet.  The shape of the coffin bone determines the shape of the hoof.  Most of the time the horses that have low heel long toe conformation will have a less than 50 degree bone angle with a long measurable toe lever (see below) and the opposite is true for upright club feet. Granted, horses that have overgrown unkempt feet may have crushed heels and a long toe but may have a good BA.  I feel that monitoring this parameter early in life could potentially identify feet that may have a common sequelae with regards to lameness later in life.  For example, a horse with a 42 degree BA and a 70mm Toe lever may be at higher risk of hyperextension injuries of the pastern, coffin and fetlock joint and increased tension strain on deep digital flexor tendon, and navicular apparatus when compared to a coffin bone with a lower bone angle and shorter toe lever. If we could identify this early in a horse's career and change the shoeing protocol to better manage this handicap maybe we could reduce the  amount of wear and tear to some degree.  

                Palmar angle (PA) also known as solar angle of the distal phalanx or ventral angle is measured from the wings of the coffin bone in comparison to a level ground surface or embedded wire in block.  It can be tricky to measure in some feet with considerable bone remodeling.  Using the wings will offer the  most consistent measurement. This gives us a manner in which to evaluate flexor tendon engagement. In general lowering the PA increases tendon tension and raising should decrease the tension. This angle will average 3-5 degrees in the horse that maintains adequate sole depth and is free of lameness but can vary greatly.  PA should be evaluated in this manner:  Is this PA healthy for this foot?  The answer comes from evaluation of sole depth, clinical exam and digital alignment.  For example, PA measures 8 degrees and maintains a SD of 15/3 and good digital alignment.  This case is higher than what is ideal but currently considered healthy for this case.  On the other hand PA measures 3 degrees and sole depth is 7mm.  This is not likely a healthy PA as a higher PA with less deep digital flexor tendon tension will unload the solar corium and vital growth center of the sole.  This angle is also of great value to monitor in a preventive podiatry program.
Tendon Surface Angle (TSA) is measured on this distal part of the navicular bone compared to a level ground marker. This is relative to the course of the deep digital flexor tendon takes at turns to attach to the coffin bone.  Monitoring the change of TSA with your applied orthotic is of value especially cases that show navicular bone lesions in this region.   Simply changing DB  may be beneficial in many cases however raising PA and TSA is often required to be therapeutic.

Figure 4 Pre and post shoeing measurements

Figure 4 illustrates the measurements that changed in a pre and post shoeing lameness case. Note the shortened shod toe lever, increase in tendon surface angle and palmar angle.  Digital alignment has improved greatly.  By raising the palmar angle and reducing the lever arm we have reduced tension in the deep flexor tendon.  Subsequently reducing load in the solar corium under tip of the coffin bone, navicular apparatus and extraction forces at the dorsal wall to sole interface.  Horse was sound and off bute the next day.

Figure 5 low versus high on a 6 mo old foal

Figure 5 is  a 6 month old fold that was being evaluated for management of a club foot.  Note the significant differences in coffin bone angle, palmar angle, and toe lever.   The main difference is the deep flexor suspension.   Each have very different mechanical properties and require different trimming and shoeing approaches.  Trying to match feet with a perfect toe angle doesn't make since when the internal structures are ghastly different.

Figure 6 long toe low heel/neg pa hoof versus a grade 3 club

Figure 6 is comparing measurements in two different adult horses.  One with severe negative palmar angle and the other a grade 3 club.  Note the difference in bone angle, palmar angle, tendon surface angle and the static toe lever.  These differences must be considered and exemplifies the reason that all hooves can't be shod the exact same way and expect it to fit all the different foot types.  It is similar to asking us all to wear the same size pants even though we all have our unique characteristics.

            Clinical evaluation is directed at evaluating growth rings, hoof quality and length from the widest part of the foot forward.  Clubby feet with higher deep flexor tension will have growth rings that are narrow at the toe and get wider towards the heel.  Again this is secondary to the loads creating a vascular compression and decreased nutrient flow to these areas.  This widest part of the foot which correlates very close to the center of rotation will typically be in the middle of or just in front of the middle of the hoof.  The low heel type foot with lower suspension properties within the deep flexor tendon will have more load in the heels.  This results in growth rings that are wider at the toe and narrower at the heel.  The widest part of the hoof is typically in the palmar third creating a long lever arm. 

Figure 7 Grade 2.5 club with heel outgrowing toe versus a crushed heel with toe outgrowing heel

                Note in figure 7 the club foot on the  has growth rings that diverge from toe to heel and the crushed heel  diverges from the heel to the toe. 

Other aspects to consider from the solar view is frog characteristics.  Typically with upright clubby feet that present as adults will have recessed atrophied frogs compared to the crushed heel hoof that will have a robust strong frog.  Paying close attention to bulges will also further define regions of excessive load.  This will occur around the apex of the frog with higher grade clubs and laminitic feet and just under the wings in negative palmar angle or crushed heel feet.  Another good tool is watching your patients go in soft footing and watching what the coronary band, toe and heel does.  Watch the heel and toe for sinking into the forgiving footing.  Watch the coronary band.  Does is stay level, rotate forward and more positive or backwards and more negative.  This will also give you a good indication of the deep flexor system and what is will allow. 

            This mechanical scenario has implications that must be considered in every foot disease.  Using the podiatry style radiograph and venogram to determine compromised areas and design a therapeutic shoeing program is paramount to have repeatable success.  Simply altering the toe lever length by setting a shoe back, rolling or rockering the toe and use of natural balance shoes has proven to offer mechanical advantage but has its limitations.  Greater success is obtained by altering and monitoring sole depth, palmar angle, tendon surface angle and digital alignment.  Difficult cases in my practice have pre and post shoeing radiographs at every visit.  The pre-shoe gives you information regarding how the horse responded to your mechanical therapy with regards to palmar angle, sole depth and digital alignment. The post shoeing radiographs sets a new baseline and confirms you have accomplished your therapeutic goal with your trim and shoe application.