Two complications after total hip arthroplasty (THA) are hip instability and limb length discrepancy; instability is a common indication for revision THA. The goal of a successful THA is to maximize impingement-free range of motion, recreate appropriate offset, and equalize limb length discrepancies to produce a pain-free and dynamically stable THA. In this article, the patient risk factors for dislocation and limb length discrepancy, key elements of the preoperative template, the anatomic landmarks for accurate component placement, the leg positions for soft tissue stability testing, and the management of postoperative instability are reviewed.
Key points
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Patient risk factors for dislocation include obesity, increasing age, neuromuscular and cognitive disorders, alcoholism, and a previous diagnosis of femoral neck fracture.
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Clinical evaluation includes gait assessment, measurement of true and apparent limb length discrepancies, and identifying a fixed or flexible pelvic obliquity and any periarticular soft tissue contractures.
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Preoperative radiographs help calculate limb length differences and plan intraoperative lengthening, component sizing, and acetabular and femoral component position in relation to radiographic landmarks.
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Larger femoral head sizes (≥32 mm), elevated or lipped liners, high offset stems, and dual mobility devices are implant options that may improve hip stability in higher-risk patients.
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Postoperative hip instability can usually be managed with closed reduction. When this fails, surgical management includes increasing femoral head size, increasing soft tissue tension with higher offset or limb lengthening, and component revision with possible conversion to a dual mobility or constrained liner.
Background
Total hip arthroplasty (THA) reduces pain and improves function in patients with end-stage arthritis of the hip and is associated with a high satisfaction rate and a low incidence of complications. Two complications after THA are hip instability and limb length discrepancy, and between 2000 and 2007, instability was the most common indication for revision THA. In addition, significant limb length discrepancy after THA is a cause for patient dissatisfaction and possible litigation. The goal of a successful THA is to maximize impingement-free range of motion, recreate appropriate offset, and equalize limb length discrepancies to produce a pain-free and dynamically stable THA. The objective of this article is to review the patient risk factors for dislocation and limb length discrepancy, key elements of the preoperative template, the anatomic landmarks for accurate component placement, the leg positions for soft tissue stability testing, and the management of postoperative instability.
Background
Total hip arthroplasty (THA) reduces pain and improves function in patients with end-stage arthritis of the hip and is associated with a high satisfaction rate and a low incidence of complications. Two complications after THA are hip instability and limb length discrepancy, and between 2000 and 2007, instability was the most common indication for revision THA. In addition, significant limb length discrepancy after THA is a cause for patient dissatisfaction and possible litigation. The goal of a successful THA is to maximize impingement-free range of motion, recreate appropriate offset, and equalize limb length discrepancies to produce a pain-free and dynamically stable THA. The objective of this article is to review the patient risk factors for dislocation and limb length discrepancy, key elements of the preoperative template, the anatomic landmarks for accurate component placement, the leg positions for soft tissue stability testing, and the management of postoperative instability.
Clinical patient evaluation
Patients at risk for postoperative limb length discrepancies include those with previous surgery, trauma, infection, growth plate arrest, and congenital dysplasia. Patients at risk of instability include those with hyperlaxity, connective tissue or neuromuscular disorders, a diagnosis of femoral neck fracture, avascular necrosis, increasing age, alcoholism, obesity, and female sex.
Patients at risk for true or perceived postoperative limb length discrepancy
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Operative leg longer preoperatively
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Perception of limb length equality when operative leg shorter (block testing)
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Significant (>3 cm) limb length discrepancy
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Fixed pelvic obliquity
Physical examination includes a gait assessment for signs of spasticity or imbalance. The axial skeleton should be assessed for coronal or sagittal plane deformities, such as scoliosis or ankylosing spondylitis. Pelvic obliquity can occur from a spinopelvic deformity or be compensatory and secondary to a limb length discrepancy or soft tissue contracture. If a pelvic obliquity is present, evaluate the patient in both the standing and seated position. A flexible pelvic obliquity corrects in a seated position, whereas a fixed pelvic obliquity does not.
Key physical examination findings
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Gait: spasticity or imbalance
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Fixed or mobile pelvic obliquity
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Soft tissue contractures (flexors, abductors, adductors, external rotators/capsule)
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Abductor strength
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Distal sensation and proprioception
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Actual or perceived limb length discrepancy
Supine range of motion testing should identify any hip or knee contractures that may affect accurate limb length assessment. Abductor strength is tested and should be compared with the contralateral side. A distal sensory examination may identify the presence of a peripheral neuropathy, which may increase the risk of gait imbalance, falls, and subsequent postoperative instability.
Limb length assessment
True and apparent limb length are the 2 methods used to assess limb length discrepancy. True limb length is measured from the anterior superior iliac spine to the medial malleolus. The apparent limb length is measured from the umbilicus to the medial malleolus. True limb length represents the length of the limb, whereas the apparent limb length takes into account all factors that contribute to differing leg lengths, such as a pelvic obliquity or soft tissue contractures. For this reason, the apparent limb length is a better reflection of the patient’s perception because it includes the true limb length difference in addition to any factors that altogether contribute to leg length inequality. Block testing with blocks of different heights placed under the affected extremity helps to quantify the apparent leg length discrepancy ( Fig. 1 ).
Preoperative templating and radiographic measurements
Standing anteroposterior (AP) pelvic and operative hip radiographs, in addition to a frog-leg or cross-table lateral, are helpful for preoperative planning. A line drawn across 2 fixed reference points on the AP pelvis view and measured from a femoral reference point (lesser trochanter) allows calculation of a radiographic leg length discrepancy. The 3 pelvic reference points include the inferior aspect of the obturator foramen, the ischial tuberosities, and the acetabular teardrop. The teardrop is the most reproducible and accurate when calculating limb length discrepancy but if distorted anatomy makes identification difficult, then another pelvic reference point can be used. Fig. 2 A shows a pre-operative AP pelvic radiograph and a leg length discrepancy as measured from the teardrop to the midpoint of the lesser trochanter bilaterally.
Preoperative templating
Tips for Preoperative Templating
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Determine any leg length differences and calculate planned lengthening
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Acetabular component
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AP radiograph: teardrop, ilioischial line, superolateral acetabulum
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Measure lateral overhang with cup in desired inclination
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Femoral component
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Component sizing for canal/metaphyseal fill
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Standard versus high offset stem
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Position femoral head center of rotation (COR) above acetabular COR to match planned lengthening
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Measure neck resection in relation to lesser trochanter
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Femoral head COR in relation to the tip of the greater trochanter
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The most helpful radiographic landmarks for acetabular templating are the teardrop, the ilioischial line, and the superolateral acetabulum. The inferior aspect of the cup should be positioned at the level of the radiographic teardrop and medialized until close to the medial wall (see Fig. 2 ). The acetabular COR affects both leg length and offset, and accurate restoration of the native acetabular COR optimizes hip biomechanics. The amount of lateral cup overhang from the superolateral aspect of the acetabulum is a helpful intraoperative check for appropriate inclination angle. The femoral component COR should be positioned in the location that best achieves leg length equality and restoration of offset. If the femoral COR is proximal to the acetabular COR, then, the leg is lengthened, and if the femoral COR is medial to the acetabular COR, then, hip offset is increased. High offset or standard femoral stem designs allow for adjustments in offset to match patient anatomy. Once the femoral COR is in a position that best restores leg length and offset, the level of the neck osteotomy is marked and measured in millimeters from the lesser trochanter. Another preoperative method for reproducing the femoral COR is to draw a line perpendicular to the tip of the greater trochanter and mark where it passes through the femoral head. This distance can then be checked intraoperatively once the trial femoral stem is in place. Restoring offset is necessary for overall hip stability and soft tissue tension. If offset is not restored first, then the leg may be inadvertently lengthened to achieve overall hip stability.
Each surgical approach has different advantages and disadvantages in regards to leg length and offset recreation. For the direct anterior approach, the patient is usually supine, which allows for leg length comparison using the medial malleoli as a guide. This comparison is not possible when a fracture table is used, because the feet are secured in traction boots. With fluoroscopy, leg length and offset can be assessed with an AP radiograph of the operative hip and compared with a radiograph of the contralateral side. The direct lateral approach can be performed either supine, with the ability to measure the medial malleoli, or in the lateral decubitus position, in which the tip of the greater trochanter can be easily used as a guide for femoral COR reconstruction. For the posterior approach, the lesser trochanter is the key landmark for making a neck osteotomy of the desired length based on the preoperative template. For patients in the lateral decubitus position, with the knees in the same position, the heels can be assessed clinically and compared with heel position before skin incision.
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