Total Hip Arthroplasty (THA), commonly known as hip replacement surgery, is a highly successful procedure designed to alleviate pain and restore function. A significant concern following this surgery is the development of a Leg Length Discrepancy (LLD), where one leg appears or feels longer or shorter than the other. This difference occurs due to factors related to the pre-existing condition and the surgical process itself. Understanding the causes and effects of LLD is important for managing expectations and achieving a successful recovery.
Understanding Leg Length Discrepancy After Hip Replacement
Leg length discrepancy after total hip replacement arises from two categories: anatomical and surgical factors. Before the operation, severe arthritis often causes the affected leg to shorten due to the loss of cartilage and bone. When the surgeon restores the joint space to its correct anatomical position during THA, the leg is brought back to its original length, which can create the sensation of lengthening.
Pre-existing conditions, such as fixed contractures or muscle tightness surrounding the hip, can mask the true length difference before surgery. These anatomical factors mean that even a technically perfect operation may initially result in a perceived difference as the body adjusts. The second category involves surgical factors, primarily related to the placement and sizing of the prosthetic components.
The surgeon must balance restoring equal leg length with ensuring the new joint is stable to prevent dislocation. To enhance joint stability and tension the surrounding soft tissues, the surgeon may use a slightly longer implant or position components to intentionally lengthen the leg slightly. Stability is often prioritized over absolute length equality because an unstable hip presents a greater risk of failure. Therefore, slight lengthening of the operated limb is a common occurrence, sometimes considered a necessary trade-off for long-term joint health.
Physical Consequences of Uneven Leg Length
When a leg length discrepancy is present, the body compensates, leading to biomechanical and postural changes. One noticeable consequence is an altered gait, often perceived as a limp, which requires increased energy expenditure for walking. Patients may find it difficult to walk long distances or stand for extended periods.
The discrepancy introduces secondary pain and strain in areas away from the hip joint. Lower back pain is a frequent complaint, resulting from the pelvis tilting or rotating to accommodate the difference. This pelvic obliquity can also lead to a functional scoliosis, which is a temporary curvature of the spine as the body attempts to keep the head level.
The longer leg experiences increased impact and stress on the knee and ankle joints due to excessive force transmission. Conversely, the shorter leg may develop hip pain because the muscles are slackened or the joint is forced into an unnatural position during the gait cycle. Significant lengthening can cause stretching of nerves, such as the sciatic nerve, resulting in numbness, tingling, or weakness radiating down the leg.
Symptoms often become significant with discrepancies greater than 10 millimeters, though some patients are sensitive to smaller differences. This functional impact can lead to patient dissatisfaction and a reduced quality of life despite the successful resolution of the initial arthritic pain.
How Doctors Measure and Confirm Discrepancy
Accurately diagnosing and quantifying a leg length discrepancy requires clinical examination and objective imaging studies. Clinically, a doctor may perform a physical measurement using a tape measure from a fixed point on the pelvis, such as the anterior superior iliac spine, down to the medial malleolus. This method provides a quick assessment of the anatomical length but has limitations, as soft tissue or positioning can introduce observer error.
A more objective clinical method involves having the patient stand and placing calibrated wooden blocks under the foot of the shorter leg until the pelvis appears level. This helps determine the functional requirement for correction. However, the most precise and reliable method is radiological assessment using specialized full-length standing X-rays, often called a scanogram or orthoroentgenogram.
These images capture the entire length of both legs, allowing for precise measurement of the bony segments. The discrepancy is calculated by comparing the distance between specific reference points, such as the top of the femoral head and the center of the ankle, on both sides. While the goal is absolute equality, discrepancies under 10 millimeters are considered well-tolerated by most patients and may not require immediate intervention.
Management and Treatment Options
Management of a leg length discrepancy begins with conservative, non-surgical approaches, which are highly successful for the majority of cases. For a true anatomical shortening, a shoe lift or orthotic insert is the most common and effective treatment. The lift is placed inside the shoe of the shorter leg, or the sole of the shoe is built up externally for larger discrepancies, to restore balance.
The size of the lift is determined by objective measurement and, importantly, by the patient’s comfort and feeling of balance. Patients with discrepancies of less than 10 millimeters may only require observation, as the body often adapts over several months as muscle tightness resolves. Physical therapy plays a substantial role in managing functional discrepancies caused by muscle imbalance and contractures.
A physical therapist works to address secondary issues, focusing on stretching tight muscles, particularly the gluteals, and strengthening the core to stabilize the pelvis. This therapy helps the body adjust to the new biomechanics and reduces the compensatory strain that causes back or knee pain. This conservative path is preferred because it is non-invasive, affordable, and easily adjustable.
Surgical correction is reserved for a small number of patients with severe, persistent, and functionally disabling discrepancies, typically exceeding 20 millimeters, that have not responded to conservative care. This involves a revision total hip arthroplasty, where the original implant components are adjusted or replaced to shorten the longer leg or lengthen the shorter one. Revision surgery carries increased risks compared to the initial operation, so it is only considered when the disability from the LLD is substantial.