Computer Navigation in Hip Arthroplasty and Robotics

Rationale and Indications

By the 2000s, fixation was a certainty and confidence in the durability of the articulation was generated by the discovery of highly cross-linked polyethylene (HXLPE). Failures in total hip arthroplasty (THA) since 2000 have mostly been caused by a failure of precision of the operation within the performance of the surgeon. This is not an implication of poor surgical ability but rather is owing to the limitations on the surgeon by the criteria for successful performance of the hip replacement. Surgeons operate using their experience, instinct, and intuition, which is all that is necessary for those patients who have essentially normal anatomy. However, outliers of THA occur, which are mostly a consequence of unrecognized abnormal anatomy of the spine, pelvis, and hip construct so that implants are not in the correct position for that patient. In these patients, and sometimes because of inexperience, the version of both the femoral components and the acetabular components is misjudged.

The anatomy of hip function has become better understood in the past 10 to 15 years. Lazennac has been the leader in educating the orthopedic community about the coordinated functional performance of the spinal-pelvic-hip construct. Normally, the hip only flexes up to 70 degrees when one sits and the rest of the flexion is a posterior tilt of the pelvis combined with a straightening of the spine of 20 degrees to 30 degrees. When one bends forward, the hip flexes, on average, 85 degrees while the femur internally rotates 12 degrees and the pelvis again posteriorly tilts. The pelvis must posteriorly tilt when the hip flexes to biologically open the acetabulum for clearance of the femoral head and neck and greater trochanter from impinging on the pelvis. Now, the consequence of stiffening of the spinopelvic construct can be understood. With stiffness that prevents the pelvis from tilting posteriorly, the hip and femur must flex more to allow sitting or bending. This increased hip motion can cause impingement, particularly of the greater trochanter against the pelvis. The pelvis can get stuck in certain positions because of loss of mobility of the spine. Because the pelvis is normally anteriorly tilted with standing, if the spine is stuck in lordosis, the pelvis will not posteriorly tilt because the spine cannot straighten and the pelvis remains anteriorly tilted during sitting. The increased hip motion necessary for sitting can cause anterior impingement; with hip replacement, this can cause posterior dislocation. If the pelvis is stuck in posterior tilt with the spine in a “flat-back” position, the acetabulum is opened for sitting but the patient must hyperextend the hip and femur to stand straight. This hyperextension can cause posterior impingement of the greater trochanter on the pelvis and anterior instability of the hip.

These anatomic abnormalities can be present in 25% to 30% of patients. It is important to consider these abnormalities before a hip replacement to incorporate them into planning the proper positioning of the cup and the combined anteversion of the cup and stem. The margin of error for cup position is significantly reduced in the popular Lewinnek safe zone, which is 20 degrees. The margin of safe zone when there is abnormality of the normal mobility of the spine-pelvic-hip construct is closer to 10 degrees. It is easy to understand that precision of component placement becomes more important in patients with abnormality of spinal-pelvic-hip mobility; this precision of component placement and combined anteversion is easier to achieve when quantifiable numbers of component position are available at surgery. Computer navigation is not necessary for patients who have normal spinal-pelvic-hip mobility but has value in abnormal mobility. This is the reason that more surgeons are stressing the importance of personalizing the hip replacement operation rather than using the same component positions for all patients.

Femoral component anteversion can be estimated by the surgeon more easily than acetabular position because more bony landmarks are evident. The femoral neck proximally can be seen, as can the epicondyles distally, to judge the axis of the femur. With cemented stems, because the stem is smaller than the canal and can be manipulated inside the canal, anteversion can be more precisely controlled by the surgeon. Errors are still made, as was evidenced in two reports. Surgeons have much less control with cementless stems, which are press-fit in a rigid bony structure. The bony structure itself varies from retroversion to high degrees of anteversion. Insertion of a stem is controlled by femoral bony neck anteversion, the anteroposterior (AP) isthmus at the level of the lesser trochanter, and of the diaphyseal bone by both external radius and the internal thickness of the posterior cortex (bone types A, B, and C ). In fact, estimates by the same experienced surgeon have the same precision for stem anteversion (11.3 degrees) and cup anteversion (12.3 degrees). This precision means that the estimate can be wrong by these many degrees. By contrast, the precision of computer navigation in the same studies was less than 5 degrees for both stem and cup.

The most important technical factor in cup position is center of rotation (COR), which should be within 2 mm of the anatomic center with cemented cups and within 3 mm superior and 5 mm medial for noncemented cups for the greatest durability of the arthroplasty. The COR can have the most influence on impingement: if it is lateralized, the metal neck can impact the edge of the metal shell (risk increases with poor combined anteversion); if it is superior or medialized, the risk of bone-on-bone impingement increases unless femoral offset, or hip length, is increased (by the use of an offset stem or lengthening of the hip/leg).

The inclination and anteversion of the cup also have rigid criteria for success. Inclination cannot exceed 45 degrees for optimal wear ; when it exceeds 50 degrees, it causes runaway wear with metal-on-metal bearings and increases the risk of breakage of ceramic or HXLPE bearing surfaces. Cup anteversion is dependent on stem anteversion because combined anteversion is the most important anteversion. The safe zone of combined anteversion is 25 to 50 degrees (37 ± 12 degrees) with men's safe zone usually lower than women's. For all hips, it is best if the combined anteversion can be kept between 30 and 40 degrees, but those with a stiff spinal-pelvic-hip construct may have higher anteversion, above 40 degrees. The reason that the femoral preparation is done first with use of a cementless stem is to determine the stem anteversion; the cup anteversion should then be created to provide the desired combined anteversion. Our experience is that cup anteversion should almost never be below 15 degrees and inclination should never be below 35 degrees. When cup position is below these numbers, the risk for impingement and instability is significantly increased.

Current technical considerations of THA require preoperative knowledge of the spinopelvic mobility, and the sequence of intraoperative preparations is determined by either cemented or cementless fixation. With cemented fixation, the surgeon can implant the acetabulum first because the femur can be adjusted in the femoral canal. With cementless fixation the surgeon should prepare the femur first so that the cup can be adjusted to the femur and the combined anteversion is kept at 30 degrees or above. Because the margin for cup positions is only 10 degrees in patients with spinopelvic abnormality, we prefer the use of computer navigation in the operating room for these patients. For the experienced surgeon who can accurately judge the cup position, computer navigation is not necessary for patients with normal spinal-pelvic-hip mobility. For inexperienced surgeons, it is still of benefit to have quantitative knowledge of component position. It has been shown that the number of dislocations is increased when hips are placed outside the Lewinnek safe zone, which is a 20-degree margin of error. For those using computer navigation, technique is important.

Technical Considerations