Leg Length Discrepancy After Hip Replacement
Functional leg length discrepancy occurs when the patient perceives the limb to be shorter or longer without actual change in limb length. Majority of the leg length discrepancies after hip replacement surgery resolve in 3-6 months.
Total hip replacement is a common orthopedic surgery and the complications associated with the surgery are rare. The technological leaps in design and the techniques of the surgery have ensured that the modern implants last 20 years or more and in some cases a lifetime.
The surgery is most commonly performed to relieve the symptoms of hip joint arthritis. During the surgery, the surgeon gives an incision at the back of the hip joint and exposes the joint after cutting/separating the various tissues. The surgeon then removed the arthritis head of the femur and cleans the acetabular socket to remove the arthritic bone.
Implants used in hip replacement surgery are made of metal alloy and plastic parts. The implants are positioned in a way to:
- Correct any deformity associated with the joint prior to surgery.
- Achieve maximum biomechanical stability to prevent artificial joint dislocation.
- Achieve a natural range of motion of the prosthetic hip joint.
The arthritis of the hip joint slowly destroys the joint over years. The destruction of the hip joint involves the destruction of both the articular cartilage and the head of the femur along with the acetabulum. The destruction may lead to a shortening of the length of the involved limb.
The body compensates for the shortening of the leg due to arthritis by tilting the pelvis down on the involved side. The tilting of the pelvis falsely lengthens the leg and the patients may not perceive any leg length inequality.
The tilting of the pelvis for a long time however leads to contraction/tightening of the muscles around the pelvis and the hip joint. During the surgery, the surgeon replaces the diseased ends of the hip joint and restores the leg length equal to the opposite normal leg. However, due to the preoperative tightness of the muscles on the operated side, the pelvis continues to be tilted down.
The continued tilt of the pelvis after the surgery leads to a perception of the operative leg to be long after the surgery. The discrepancy responds very well to stretching during physical therapy and most patients would have a resolution of the leg length discrepancy in 6 months.
The leg length discrepancy may also occur due to intraoperative techniques. During the surgery, one of the goals is to create a stable prosthetic joint. The stability of the joint is necessary to prevent hip joint dislocation in which the ball of the joint pops out of the socket.
The stability may require a long neck of the prosthetic stem or a larger size of the prosthesis. The increase in the length of the joint increases stability but may cause the operated leg to be longer. The lengthening of the leg during surgery may also occur due to a mal-positioned prosthetic cup.
Excessive soft tissue release to counter deformity may cause instability and the surgeon may use a prosthesis with a long neck to increase offset that may cause leg lengthening.
Similarly in an unplanned total hip replacement, the use of smaller implants may cause shortening of the operated leg. The leg may also appear shortened to the patient who may have an upward pelvic tilt (pelvic obliquity).
The patients may complain of the perception of the operated leg to be longer or shorter and may walk with a limp. The patient may also complain of an associated pain that may get worse with activity.
In cases, when there is the true lengthening of the operated leg, the lengthening may stretch the nerves in the leg such as the sciatic nerve, peroneal nerve, or the femoral. The shortening or lengthening may put additional stress on the lower back leading to lower back pain.
The stretching of the nerves may cause symptoms of paresthesia, numbness, or weakness in the leg, thigh, or foot in extreme cases. Similarly, when there is true shortening, there may be reduced muscle tension in the hip region that may cause weakness of the muscles. Without adequate muscle tension, the patient may walk with a limp and the prosthetic joint may dislocate easily.
The most common cause of leg length discrepancy is pelvic tilt due to tightening of the muscles (pelvic obliquity). The apparent leg-lengthening responds very well to stretching exercises and the patient’s pelvis comes back to the normal position after adequate stretching and exercise. The majority of the patients report complete resolution of the symptoms in 6 months.
Patients with a true shortening of 10 mm may not need any treatment as the 10 mm difference is well tolerated by the body (through tilt in the pelvis and the spine) and the patient may report resolution of symptoms as the body adapts.
Rarely, patients may need a shoe insert or a shoe lift after 6 months of the surgery. The shoe insert or the shoe lift is added to the shorter leg. The surgical management consists of revision hip replacement but is extremely rare.
The surgical management is only done when all other methods have failed. The revision surgery involves the use of a longer or shorter implant or may involve correction of the implant position.
Prevention and Summary
With the use of modern techniques and implants, leg length discrepancy after hip replacement is extremely rare. Templating the hip before the surgery involves the measurement of the head size, neck size, and position according to the normal hip.
The planning of the surgery with templating helps reduce the possibility of wrong size implant and leg length discrepancy. Modern hip replacements involve the use of robotic arms and intraoperative CT scans.