The annual number of primary hip replacement surgery is rapidly growing in the U.S. due to the continued rise of the obese population. A primary total hip replacement (THR) replaces the entire hip joint. THR implants typically have three components: a stem, a head and an acetabular cup. The stem anchors the implant in the femur and the portion of the stem that emerges from the femur and attaches to the head is known as the neck. The head is ball-shaped and replaces the natural head of the femur. It articulates with the cup, which sits in the acetabular fossa. THR implants are either sold as one-piece (monobloc) or modular designs. In a modular implant, the head, neck and stem can be separated, replaced or adjusted for greater versatility in matching the system to a patient’s body. The greater majority of THR implants sold are of the monobloc variety. This is driven by a desire for expediency during the surgical procedure. However, the advantage of a modular design is also its downside because the device adds complexity and time to surgical procedures. The second reason for surgeon preference for one-piece implants is because of concern for a greater risk of infection because of the increased number of surfaces in modular systems compared with one-piece systems.
Partial Hip Joint Replacement
Partial implants replace only the femoral or acetabular side of the joint. This form of procedure is known as hemiarthroplasty. These devices are colloquially known as hemis within the industry and are often modular components of total implant systems. Partial implants are divided into devices that have either unipolar or bipolar heads. In unipolar implants, the head is fixed rigidly to the neck of the implant, and in bipolar systems, the head is hollow and articulates on the neck shaft of the prosthesis even as it articulates with the pre-existing cartilage on the acetabulum. Although the design is more complex, a bipolar arrangement produces less stress on the cartilage lining. Partial implants are also useful for the treatment of hip fractures. In some cases, the devices may be simpler and more effective to use than trauma-fixation hardware.
Resurfacing Hip Joint Replacement
Resurfacing is an alternative form of implantation to total joint replacement. Rather than removing the entire femoral head, a cap with a short stem is placed onto the bone. This allows for greater preservation of bone stock over a THR and also has the advantage of being easier to revise when the resurfacing implant becomes worn. Recent studies have suggested that initial revision rates between hip resurfacing and total joint replacement is not dissimilar, which is a dissuading factor against resurfacing when surgeons and patients choose between the two procedures.
Revision Hip Joint Replacement
Revision implants become necessary when original implants of any type become worn over time or through pathology. A conservative estimate of the viability of any implant is approximately 15 years. Revision devices are not significantly different than the original primary total replacements, but the stems are longer to aid fixation within the femur. Revision surgery is more involved than primary surgery and is complicated by the older average age of patients involved or in cases where infection leads to the failure of the implant. The standard of care for hip revision surgery is to use a two-stage approach, where the worn implant is removed and replaced with a molded bone cement spacer that is preloaded with antibiotics. This is kept in the patient to maintain tissue tension until all traces of infection are cleared. Afterwards, the spacer is removed and a new device is implanted.