Hip Replacement - Posterior Lateral Approach
Total hip replacement is an operation which provides a new joint surface and substitutes for the lost cartilage. In the hip it is called total hip replacement, because it totally replaces the surface of the hip. Most commonly, this operation is performed by removing the bony ball and replacing it with a metal ball. The metal ball is held in the correct position by a stem (or rod) that goes into the middle of the femur (thigh bone). Most often this stem is permanently fixed to the body by bone ingrowth of the patient's bone into the metal. The metal has a rough surface which fools the bone into attaching to the metal. A second method of fixation to the bone is bone cement. This was the original fixation method. It has had results for as long as 20 to 25 years with excellent success, and is still commonly used in patients that are aged 75 or older. The advantage of cement fixation is that patients do have a more rapid functional recovery because the fixation is immediate.
In the acetabulum (pelvic side of the hip joint), a metal shell is placed against the bone and again, the bone grows into the rough surface of the metal. This gives permanent fixation. Almost all acetabular components (cups) are fixed by bone ingrowth today. Originally all were fixed with bone cement. That fixation is still preferred by some surgeons, again, for patients that are 75 years or older. Into the metal shell is placed a liner which locks into place. This liner is most commonly plastic. Currently the plastic being used is called a highly cross-linked polyethylene. The one we use is named Durasul. The other choices for liners into the metal shell include one which has a metal face so that when it articulates with the metal ball there is a metal-on-metal bearing, which is named Metasul. The third choice is a ceramic liner which would mate with a ceramic head to give a ceramic-on-ceramic bearing surface. This articulation surface is currently not in use at the Arthritis Institute. The two we use are metal-on-cross-linked polyethylene and metal-on-metal.
The size of the ball used for hip replacement can vary between 22mm and 44mm. One determinant for the size of the ball used is the size of the acetabulum of the patient. Every liner in the metal shell requires a certain thickness for longevity. Clearly then, a smaller acetabulum requires a smaller head to permit a thick enough liner in the metal shell for the hip to last many years. The metal head choices for metal-on-metal are 28mm and 32mm. The head size choices for Durasul are 22mm through 44mm.
The healing for the bone into the metal of the total hip replacement (bone ingrowth) requires six months to 18 months for complete maturity. This is the time required for bone to completely remodel after it is injured during the performance of the operation. The remodeling of the bone occurs by the cells of the bone reforming the structure of the bone (named osteon remodeling). However, the time for complete maturation of the bone healing does not mean that the patient cannot be functional on the hip during the healing time. The immediate fixation of the implant is obtained by a press fit into the bone, and this press fit is strong enough for the patient to walk on the bone immediately and to be functional for all activities of daily living immediately. Sporting activities require some healing before being done. Less vigorous activities such as golf can usually be played at 2 to 3 months. Certainly, exercise and gym work can begin early. Riding an exercise bicycle is permissible immediately after the surgery. However, for more vigorous activity such as skiing or individual sports such as tennis or racquetball, the waiting period should be six months, to allow the maturation of the bone to be in process.