There are approximately 300,000 total hip replacement surgeries performed annually in the United States with a 98% success rate with satisfied patients. Osteoarthritis (wearing out of the joint surface in this ball and socket articulation) which can cause groin, buttock, and thigh pain is the indication for hip replacement surgery in 85% of patients. The other 15% of hip surgeries performed are in response to less common conditions, such as rheumatoid arthritis and bone necrosis (death) of the femoral head. Because implant designs have improved, replacement of the diseased hip joints in patients 50 years and older is not unusual, 20 years after the surgery over 85% of implants remain intact and functioning well. Not only does this procedure eliminate most, if not all, of the preoperative pain, in most cases it allows the patient to return to very active lifestyles, including skiing, tennis, and cycling.
There have been bumps and potholes along the road to the present day success of this remarkable scientific achievement. It began over 50 years ago, pioneered by a British orthopedic surgeon, Sir John Charnley, who doggedly pursued the principle of removing the diseased hip ball and socket and implanting nonliving materials to function as the joint. Charnley was knighted in 1977 (in the same ceremony as Paul McCartney, though McCartney’s knighthood was for entirely different reasons!) by the Queen of England for his achievements.
An early obstacle to hip replacement surgery was biocompatibility — finding and preparing materials that could be permanently implanted without being rejected by the body. Other challenges included finding a method of securely fixing the implant material to the host bone without it coming loose as well as recreating the low friction yet durable biologic articular cartilage joint surface. A great deal of effort went in to identifying the optimum material to use and Charnley settled on stainless steel which could be fabricated into a shape that prevented the implant from breaking under repetitive loads. Today through successful trial and error, most of these hurdles have been overcome, but 30 years ago it was not uncommon for a new innovation, such as using Teflon to coat the metal implant, to fail miserably requiring removal of the device within a year.
Over time, the weak link in the construct was discovered to be the bearing surface between the ball or femoral head replacement (about the size of a billiard ball) and the hemisphere cup attached to the pelvis. High density polyethylene evolved as the bearing surface with low friction and acceptable durability on the cup side, which articulates with the metal ball. However, with long-term follow up studies, it was determined that slow wear of the polyethylene (also used in sewage pipe) resulted in tiny plastic particles seeping into the interface between the metal implants and neighboring bone causing a local reaction and eventually loosening of the metal implants resulting in implant failure. Over the past 10 years ceramic, which is a hard brittle-like glass that does not wear, became an acceptable alternative. An unforeseen side effect of this ceramic-bearing surface was an irritating, loud, high-pitched squeak, “the squeaking hip” that occurred in about 5% of patients and often required revision surgery.
Another recent innovation, known as the metal hip, has caused quite a stir in the news media, lawyer ads, and cocktail party conversations because of a devastating complication that took five years to appear. In Dr. Charnley’s day, metal could not be used as the bearing surface, because of its poor frictional properties. With recent improved technology, metal could be finely polished smoothing its surface and reducing friction to an acceptable range. Metal does not wear, does not squeak, and does not shatter like glass. It had all the advantages and many were implanted. Then it was discovered that miniscule metal wear particles in the joint caused a local tissue reaction that silently destroyed all the surrounding tissue, bone, muscle, and ligaments making revision surgery very difficult. Even though this has occurred in only a small percentage of metal hips, what was thought to be the bearing solution is now rarely, if ever, used.
In the meantime, the old standard, high density polyethylene has been upgraded by irradiating the plastic and improving its durability and increasing its longevity by 400%. It remains the most effective and widely used bearing material.
The scientific, innovative mind continues to march on, striving to improve on this remarkably successful orthopedic procedure. Our culture seems bent on obtaining the latest and greatest, but you may not want to be the first on your block to try the “latest and greatest” total hip replacement, until it’s had a chance to undergo some testing in the field
Jon Thatcher, MD is a board-certified orthopaedic surgeon at BMH Orthopaedics & Sports Medicine, a department of Brattleboro Memorial Hospital. Dr. Thatcher can be reached at 802-251-8611.