Hip Biomechanics: Part 4 of 6

This diagram (Figure 4) also shows a balanced equation; HAM x D1 = D2 x BW.  (where HAM=hip abductor/g.medius, D1 and D1 are lever arms, BW= body weight). This is not exactly a desirable scenario or strategy to obtain when it comes to joint compression mechanics however it is close to representing what is accurate in the human hip.  Since the gluteus medius muscle is the primary joint compressor in the frontal plane (it applies two thirds of the compressive forces across the joint) we would ideally never want such a large HAM force.  Typically the internal to external moment arm ration is 2:1 thus this model would require a HAM force twice the body weight to maintain a balanced system.  None the less, we would want to offset these forces somehow.  The only way to offset the large HAM would be to move the pivot point closer to the BW thereby increasing the D1 (increase D1 and you can reduce HAM and thus joint compression load).  In a physical person the pivot point, the joint axis of movement, is fixed so there is no real strategy to improve the situation without surgery.  These patients are unlucky and have no strategies to improve their high compression forces unless they loose weight; due to the fact of the 2:1 ratio, for every pound of body weight loss there is a 2 pound force decrease in the HAM.  Obesity is going to wreak havoc on our populations hips.

As mentioned previously, the model presented is very much incomplete.  Muscular forces surround the joint, movement occurs in every cardinal plane and there is acceleration of body segments which requires even greater muscular contraction isometrically, concentrically and eccentrically.  These factors all considered, it has been calculated that the total hip force crossing the joint can reach 3 times the body weight during walking.   This force is welcomed for maintaining joint stability but it can be an unwelcome force in a degenerative arthritic joint where the cartilage is less pliable and flexible.  The loading forces in an arthritic joint rhythmically pass into the acetabulum and femoral head as a result of the compromised cartilage necessitating increased bone mass and sclerosis within them.  This compromised arthritic joint will have some minor laxity due to the loss of the cartilage bulk and thinning of the acetabular labrum.  Thus the joint will have a slight increase in translatory/accessory movement and require greater muscular contraction to minimize/stabilize these movements.  These increased forces will be unwelcomed as they will generate more pain.  Additionally, the increased movements and degenerative debris within the joint will cause irritation and inflammation of the joint capsule and synovial lining causing further pain.  This entire scenario will cause the patient to investigate conscious and subconscious gait strategies to reduce the compression across the joint, in other words, they will essentially seek gait strategies that will reduce HAM (gluteus medius contraction) and increase the D1 internal moment arm.  These strategies will reduce the perpendicular joint compression forces that likely will be causing pain but if performed well they will be devastating to the normal frontal plane equilibrium since the gluteus medius muscle will be essentially shut down and inhibited.  Thus, the patient’s gait strategy will give us the compensated Trendelenburg gait pattern.  The uncompensated Trendelenburg gait will show a dropping of the contralateral hemipelvis on the swing side during gait, this is the pathologic gait pattern we see when the patient has not implemented strategies to reduce their pain but it is more likely seen when the patient is not yet at the painful stage in which they need to implore strategies to avoid the movement.  Comparatively, compensated Trendelenburg gait pattern will display a lifting of the contralateral hemipelvis.  This strategy is not implemented by activation of the gluteus medius on the side in question, rather it is a compensation move performed by shifting the patient’s body weight over the pathologic hip thus causing the hip that is dropping to be passively raised into a more normal range in the frontal plane.  This passive frontal plane move by the patient over the painful hip is at first difficult to embrace logically as one does not expect to want to load their body weight further over top of the painful hip.  However, upon investigation of the mathematical equation one will see that the shift of body weight (BW) over the affected hip will significantly reduce the D2 external moment arm, significantly increase the D1 internal moment arm and thus deliver us the desirable significant reduction in the HAM gluteus medius compressive contraction across the painful hip.  Thus, the pathologic compensation gait pattern in the frontal plane will markedly reduce the patient’s hip pain.  From a kinetic chain perspective however, there is always a price to pay.  This implemented strategy of ipsilateral trunk lateral flexion is performed by utilization of the thoracolumbar paraspinals and quadratus lumborum on the painful hip side. The resulting abnormal muscular and joint strategies now imparted on the lumbar spine and pelvis interface frequently begins a cascade of muscular and joint pain in the low back and abnormal loading of the lumbar discs.  The strategy also begins an unwelcome increased loading of the non-painful hip as the patient is loading the hip greater than normal due to the height from which the hip and pelvis drop from the compensated Trendelenburg position.  In other words, by protecting the painful arthritic hip from increased loads we sacrifice the healthy hip for a period of years until the forced finally amount to enough damage that pain begins here as well.  Fortunately, we have the ability to mediate some of these dramatic movements and forces by using logic and a cane.  By placing a walking cane in the hand opposite to the painful hip and by asking the patient to contact the cane with the ground when they initiate contact with the painful limb we can offset some of the excessive compensations and forces.  When the cane contacts the ground the patient is to apply a mild to moderate downward force through the cane via arm contraction.  This downward force will afford us a resultant upward ground reactive force through the cane delivering us a lifting effect on the dropped hemipelvis side (dipping hip side/non-painful side).  This strategy will allow us a more passive shifting of the body weight (BW) over the painful hip side without having to lift or pull the body weight (BW) over the painful hip with the hip abductor muscles (HAM).  These passive forces (which can be more than  half of those normally needed to be generated by the HAM) will help to markedly reduce the muscular forces needed by the spinal and quadratus muscles while also rendering the desired marked reduction in HAM compressive forces across the painful joint.  It is interesting to note that the further the cane is placed from the body, the longer its moment arm and thus the less downward force necessary by the patient’s arm.  It is quite possible, that if used correctly, a cane can almost completely offset the required contralateral HAM force.  Another passive strategy would be to carry objects (purses, books, grocery bags, etc) on the affected hip side.  This action will also balance the teeter-totter  in favor and thus reduce the muscular forced necessary to perform the same task.  It must be noted however that increasing any body load is undesirable and should be avoided not so much because of issues pertaining to the painful degenerative hip but because of the increased load on the healthier hip.

Shawn and Ivo, The Gait Guys

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