The incidence of limb-length discrepancy (LLD) after total hip arthroplasty (THA) ranges between 1% and 30% depending on the criteria for what constitutes a LLD [1–4]. While some studies have reported minimal functional consequences with a LLD of up to 2 cm, other studies have found that leg length differences greater than 5 mm to be associated with decreased patient satisfaction [5–8]. Beard et al. [6] found that patients with LLD > 10 mm had significantly worse Oxford hip scores and Mancuso and Sculco [9] found LLD to be an independent risk factor for worse clinical outcome after THA. Symptomatic LLD can result in a limp, low back pain, compensatory pelvic obliquity, increased energy consumption during gait, and nerve palsy [8, 10–12]. These functional limitations, combined with patient dissatisfaction, make leg-length problems the most common cause for litigation after THA [3, 8, 13]. For all of these reasons, achieving limb-length equality during THA is important and discrepancies should be minimized. However, achieving equal limb lengths is not always feasible. For example, if the operative leg is longer preoperatively, substantial shortening at the time of THA can lead to instability. The contralateral limb may have been shortened secondary to previous trauma, infection, skeletal dysplasia, or growth plate arrest. Conversely, the operative leg typically cannot be lengthened greater than 2.5–4 cm due to the risk of sciatic nerve injury [8]. Preoperative patient education is essential in these situations to modify postoperative expectations , so the patient is prepared for a residual LLD after THA.

The preoperative patient history and physical exam should identify any extra-articular sources of LLD such as congenital lower extremity limb deformities, childhood growth plate arrest, trauma, infection, and any previous surgery. The history should also include the patient’s subjective perception of limb-length inequality and use of a shoe lift on the operative or contralateral side. A tape measure can be used to determine true (structural) and apparent (functional) LLD. The true, or structural, length of the limb is measured from the anterior superior iliac spine to the medial malleolus. The apparent, or functional, LLD is a combination of the structural difference in limb lengths in addition to any changes in pelvic position or soft-tissue contractures that affect the position of the limb. The functional LLD is measured on each limb from the umbilicus to the ipsilateral medial malleolus. Any patient perception of leg-length inequality should be quantified with block testing in which blocks of varying heights are placed below the shorter limb until the patient feels equal limb lengths. Any soft-tissue contractures around the hip and knee should be identified as this leads to a functional limb-length discrepancy.

In addition the patient and radiographs should be assessed for the presence of a pelvic obliquity. A pelvic obliquity may be primary or secondary depending on the source. A primary pelvic obliquity originates from spinal pathology, such as scoliosis in the lumbosacral spine or prior lumbosacral fusion. A secondary (or compensatory) pelvic obliquity is due to structural limb-length differences or soft-tissue contractures that result in a functional LLD. The pelvis compensates for this by tilting superiorly or inferiorly in order for both feet to be in contact with the ground. Most secondary (or compensatory) pelvic obliquities are flexible and will naturally correct once the soft-tissue contracture or structural limb-length discrepancy has been addressed. Patients with a primary pelvic obliquity more often have a fixed pelvic obliquity. The way to differentiate between a fixed or flexible pelvic obliquity is to evaluate the patient in the standing and seated position [14]. A flexible pelvic obliquity will correct (as assessed by the location of the iliac crests) in the seated position. Radiographs of the patient in the case example demonstrate a change in pelvic obliquity from the standing to seated position. With a history of lumbar fusion, it is unclear whether or not this was a fixed or flexible pelvic obliquity. EOS radiograph taken in the seated position (Fig. 9.2) demonstrates radiographic equalization of the iliac crests and confirms that the patient has a flexible pelvic obliquity that is compensatory and offsets the 1.4 cm limb-length discrepancy. Whether a pelvic obliquity is flexible or fixed affects preoperative templating and planned final limb position. A flexible obliquity should correct after surgery and does not have to be taken into consideration. A fixed obliquity will not correct and must be incorporated into the preoperative plan. In the case example, since the pelvic obliquity was confirmed to be compensatory and flexible, the right leg underwent planned lengthening to create leg-length equalization with the assumption that the pelvic obliquity would correct over time.

The radiographic evaluation of LLD prior to THA is critical to create a preoperative plan that will adequately restore limb length at the time of surgery. Limb-length differences can be measured on the AP pelvis radiograph by choosing two symmetric points on the pelvis (either the acetabular tear drop, the inferior or superior obturator, or the inferior aspect of the ischium) and two fixed reference points on each femur (most commonly the most medial or apex of the lesser trochanter) [15]. The difference between these two measurements reflects the estimated intra-articular LLD between the two limbs. In the majority of patients undergoing THA, the operative limb is a 2–3 mm shorter secondary to intra-articular cartilage loss and corrects when the native femoral head and acetabular center or rotation are restored after component placement.

Preoperative templating allows for estimation of implant size and expected component position in relation to anatomic landmarks. The projected leg lengthening at the time of surgery is equal to the difference between the projected acetabular center or rotation and the femoral head center of rotation [16]. The inferomedial aspect of the acetabular component is usually at the level of the acetabular teardrop or slightly inferior. The acetabular component should be positioned within the superior acetabular dome to the depth of subchondral bone. Next, the center of rotation of the femoral head should be marked. The femoral component that matches the proximal femoral size and shape while restoring the femoral center of rotation is selected and the location of the femoral neck osteotomy marked and measured in relation to the lesser trochanter. Figure 9.3 demonstrates the preoperative template of a patient with an 3 mm intra-articular LLD secondary to cartilage wear. With a systematic approach to preoperative templating and intraoperative execution the leg lengths can be reliably restored (Fig. 9.4). In regard to the accuracy of achieving limb-length equality, Woolson et al. [17] used preoperative templating to determine the level of neck resection and reported that LLD was less than 6 mm in 86 and 97% within 10 mm after THA.

Occasionally, significant femoral head bone loss , or severe flexion and/or external rotation contracture, can make it difficult to accurately template and measure limb-length differences and lesser trochanter to center (LTC) distances. Figure 9.5 shows an AP pelvic radiograph of a 47-year-old male with severe left-hip osteoarthritis with a 20° flexion contracture and 30° external rotation contracture (Fig. 9.5). A flexion contracture will reduce the LTC measurement, while an external rotation contracture will underestimate true femoral offset (Fig. 9.6). In these cases, the contralateral hip can be used as a guide for appropriate templating (Fig. 9.6). When bilateral soft-tissue contractures or bony deformities exist, a computerized tomography (CT) scanogram or dual-planar radiograph s can be utilized to obtain a more accurate assessment of limb length (Figs. 9.7 and 9.8). Utilization of the contralateral leg for limb-length measurements allowed for accurate limb-length restoration in this patient with resolution of his functional limb-length discrepancy (Fig. 9.9).