3. HIP STABILITY

Hip resurfacing has a lower rate of dislocation.

This is important for all patients, but particularly those that wish to engage in extreme motion activities such as yoga, ballet, and kayaking.

Dislocations in my series of 7000 hip resurfacing arthroplasties (HRA), dislocation has occurred in 0.3% of cases. Of these, 0.1% have had recurrent episodes of instability and require another operation to correct the problem. The international HRA study has very similar data. Most dislocations occur in the first 6 months, thereafter it is exceedingly rare.

For total hip replacement (THR) the instability rate is about 3% but varies highly. About 1-2% require revision for recurrent instability. Unlike HRA, Risk persists throughout life. There are several risk factors for dislocation:

1. Bearing size: The standard THR bearing is from 28mm- 36mm. The larger the bearing size the lower the risk of dislocation, but the higher the risk of trunion corrosion.
2. Approach: The anterior hip approach is associated with a lower risk of dislocation, but carries a higher risk of femoral fracture and deep infection.
3. Patient Factors: women, older patients, those with neurological deficits, and those with dysplasia carry a higher risk of dislocation.

Natural bearing sizes vary from 40mm to 60mm. The average woman has a bearing size of 46mm, and the average man has 52mm. HRA restores these natural sizes and results in a dislocation rate of 0.3%, while THR bearing sizes are usually 28-36mm and result in a dislocation rate of 3%. THR creates a biomechanically abnormal hip that is inherently unstable. Most of the HRA dislocations occur during the initial soft tissue healing process (6 months) and are due to patients failing to follow instructions. Once the tissues are healed, all restrictions are lifted with HRA, and all extreme bending activities such as yoga, kayaking, and ballet are allowed. In THR, the hip is always unstable and extreme positions anytime throughout the patient's remaining life put them at risk for dislocating.

It is well-established that the smaller the artificial bearing size compared to the patient's normal is, the higher the risk of dislocation. A larger bearing size in THR requires a thinner 2-piece cup. In the past, this led to high rates of plastic wear failure (plastic becomes too thin). With modern antioxidant-stabilized cross-linked polyethylene (VE XLPE) the wear problem has largely been solved and thinner 2-piece cups are more durable. But another problem has surfaced: trunion corrosion. The larger the artificial head, the greater the stress on the connector to the stem (trunion) and the higher the chance of a trunion corrosion failure. Current estimates are that this may occur in 1-5% of THR within 10 years. We still do not understand this problem well and therefore cannot design a solution. Therefore, in THR, the smaller the ball the higher the dislocation risk, but the larger the ball the higher the trunion corrosion risk. Even the largest balls do not approach the patient's natural bearing size. Only HRA can create an artificial joint with near-natural stability and no possibility of trunion corrosion.

But a resurfaced hip is not totally normal. An HRA is more stable than a THR even before any healing occurs. But until the muscles and ligaments that we have cut and repaired have had a chance to heal, position precautions are required (6 months). Rare dislocations after this time may be due to the fact that in some patients, the soft tissue healing process has failed in some respect, or simply due to the fact that the central ligamentum teres has been removed (in every THR and HRA) and that hip stability is not quite normal. But from a practical standpoint, hip stability in HRA is near normal at 6 months postop and virtually all range of motion restrictions are lifted. Only a few extreme yoga and ballet positions should still be avoided until 1 year after surgery.