14.1 Introduction

Understanding component positioning in total hip arthroplasty is critical to achieve a consistent, successful result. Improper component positioning can result in dislocation, limb length discrepancy, decreased hip range of motion, impingement, accelerated wear, and the need for revision. ^{1 – 3} Whereas the safe zone for component position described by Lewinnek et al in 1978 is still the standard used today, positioning of components is a three-dimensional exercise and there is no one reference that produces perfect results every time. ⁴

14.2 Preop

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  Surgical table. A regular table is generally acceptable, but special tables and extensions may be utilized in direct anterior and muscle sparing anterolateral approaches.

  
  
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  Patient position. A lateral decubitus position is utilized for most approaches including posterior, direct lateral, and anterolateral approaches. A supine position is utilized for direct anterior and occasionally muscle sparing anterolateral approaches.

  
  
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  Patient considerations. Excess lumbar lordosis and scoliosis can inhibit accurate component placement. Consistent placement of the patient on the surgical table is helpful to reproduce component placement, especially for the placement of the acetabular component.

  
  
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  Patient examination. Always obtain a detailed medical history and surgical history. Be sure to ask specifically about any history of trauma or prior surgery to the hip as well as any history of congenital, genetic, or neuromuscular conditions. Preoperative physical examination should include strength testing and a thorough neurovascular examination. The patient should also be evaluated for leg length inequality, lumbar lordosis and/or scoliosis, and soft tissue contractures about the hip.

14.3 Acetabulum—Reaming Technique

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  Any surgical exposure can be utilized.

  
  
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  Expose the acetabulum to the point that the superior dome is clearly seen. Make sure to identify the ischium and the pubis by visualization and/or palpation. Remove the labrum as well as any soft tissue from the acetabular fossa. Resect any peripheral osteophytes in order to expose the true acetabulum.

  
  
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  Begin reaming the acetabulum. The first ream should not be in contact with the superior dome, as there should be space between the dome and the reamer. If the dome is reamed, it is eccentrically reamed and the result is usually a bigger component than is necessary, uncoverage of the implant laterally and a higher hip center. The first reamer should be the same size as the native femoral head or slightly smaller. Beware of soft or osteoporotic bone and do not ream too aggressively, or the reamer can ream through the medial wall (▶Fig. 14.1).

  
  
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  Sequentially ream the acetabulum in 1 to 2 mm increments to widen the ream to the acetabulum edges. Make sure to follow the path of the first reamer to avoid eccentric reaming. Use the depth of the remaining bone in the acetabular fossa to gauge how close the reamer is to the medial wall. It is not necessary to ream all the way to the medial wall, but the reamer should be deep enough to cover the implant laterally in most cases. About 50% coverage is minimum, and bleeding bone should be the bed for the implant after reaming is completed. In most cases of primary joint replacement, coverage is nearly 100% (▶Fig. 14.2).

  
  
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  It is common to ream to 1 to 2 mm less than the final implant size; however, this depends on the patient’s bone quality and the manufacturer recommendations for the particular implant being used.

  
  
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  Insert the cup and impact it with the appropriate version and inclination. The preoperative template and/or anatomic landmarks can be used a reference to confirm proper position. If consistent in-patient positioning, some surgeons use the anterior, distal corner of the bed as an estimate of version. In this case, patient positioning on the bed is important for reference because if the pelvis is higher than the torso, decreased inclination can result inadvertently. On the other hand, if the pelvis is lower than the torso, increased vertical position may result. Likewise, if the patient’s pelvis is anterior to the torso, cup anteversion can be decreased; whereas, if the patient’s pelvis is posterior to the torso, cup anteversion can be increased.