Component Positioning and Stable Fixation in Total Hip Arthroplasty

Introduction

Total hip arthroplasty (THA) is one of the most successful and cost-effective surgical treatments performed within the field of orthopaedics and across all surgical specialties. The main goal of THA is to provide patients with a stable joint that is enduring and painless. In order to achieve such results, it is imperative to obtain adequate fixation of both the femoral and acetabular components in the proper position at the time of index surgery.

Malpositioning and unstable fixation of components are known risk factors for instability, accelerated wear, construct failure, and limb length discrepancy. ^, Postoperatively, each of these complications may lead to poor outcomes, decreased patient satisfaction, increased morbidity, and, potentially, mortality.

Diligent preoperative planning may mitigate the risks of instability and malpositioning. However, intraoperative complications and difficulties are likely to occur. Therefore, it is crucial that the adult reconstructive surgeon be aware of potential problems that may be encountered at the time of surgery and have the ability to formulate and implement a timely and effective treatment strategy.

In this chapter, we will briefly review some of the most commonly encountered difficulties and complications as they pertain to component position and stable fixation in total hip arthroplasty. We aim to provide some history and insight into the prevention and successful management of these scenarios when they are inevitably encountered throughout the career of an adult reconstructive surgeon.

Preoperative Considerations

Initial Evaluation

The importance of preoperative planning for THA cannot be understated. This starts at the level of the history and physical examination and should take into consideration patient-specific factors and conditions that may affect the morphology and version of the native hip. For example, patients with developmental dysplasia of the hip or a history of avascular necrosis of the femoral head tend to have increased native anteversion ( Fig. 18.1 ) . Femoroacetabular impingement and slipped capital femoral epiphysis patients may have increased native retroversion of the hip. All of these diagnoses may also result in migration of the native hip center, which—along with restoration of leg length—is critical when considering correct component positioning. Preoperative radiographs should be scrutinized to identify deformity, bone loss, and bone quality. These factors may have implications on which approach and which implants will be utilized at the time of surgery.

Templating

Templating should be routinely performed on all patients prior to THA. A proper template creates a visual guide for restoration of hip center, re-creation of femoral offset, and equalization of leg lengths. Multiple studies have also shown that sizing of both the acetabular and femoral components can be estimated with accuracy based on a preoperative template. ^, Implant type may also be influenced, for example, in a patient with Dorr A proximal femoral bone or a small proximal femoral metadiaphysis; smaller or micro implant sizes may be templated in order to optimize fit. Placement of the most appropriate implant of correct size plays a critical role in obtaining stable fixation in acceptable alignment intraoperatively. The senior author recommends displaying templates in the operating room at the time of surgery. An example can be seen in Fig. 18.2 , in which the operative hip was templated and implant specifications, neck cut length, and preoperative leg lengths are clearly displayed.

Implant Options

Surgeons must possess thorough knowledge and understanding of the different types of implants at their disposal when performing THA. Based on patient anatomy and bone quality, certain implants may be more indicated than others to achieve appropriate positioning and stability. Currently in the United States, the majority of primary THAs are performed using cementless femoral and acetabular components. Thus, we will mainly focus our discussion on the design and indications for these types of implants. ^,

Cementless Femoral Components

Khanuja et al. describe six general types of cementless femoral stems based off of their geometry, which dictates where fixation occurs. Understanding the shape and biomechanics of each stem can help to ensure adequate stability and positioning when used in the proper setting. In general, there are wedged designs that obtain fixation in the femoral metaphysis. There are tapered stems of slightly varied shape that obtain fixation at the metaphyseal/diaphyseal junction, and sometimes in the proximal diaphysis. Long cylindrical stems obtain fixation in the diaphysis; modular stems, which have two components prepared separately, can obtain fixation in the diaphysis as well as the metaphysis of the proximal femur.

Understanding the shape and points of fixation of these different stems allows the surgeon to preoperatively plan and template the implant that will give the best chance for optimal fixation and proper component positioning at the time of surgery. Multiple studies have demonstrated excellent intermediate and long-term survivorship of wedged stems that obtain fixation in the metaphysis when used on both old and young patients. These are the stems most commonly utilized for routine THA across the United States.

In patients that present with poor bone quality, dysplasia, or otherwise abnormal anatomy of the proximal femur, implants that achieve more distal fixation are often required. The use of tapered, long cylindrical, and modular stems that bypass the proximal femoral metaphysis and obtain their fixation in the distal metadiaphyseal and diaphyseal regions have been used with great success in complex THA ( Fig. 18.3 ). These implants allow for avoidance of dysplastic and nonsupportive bone while enabling solid initial fixation distally. These implants also allow for correction of leg length, version, and offset as necessary. The surgeon should have knowledge of specific sizes, neck shaft angles, and offset options for each implant that may be utilized. Multiple studies have demonstrated excellent intermediate to long-term survivorship of these stems when utilized in complex hip arthroplasty in patients with abnormal proximal femoral anatomy.

Cementless Acetabular Components

Historically, at the advent of THA, cemented fixation of the acetabulum was common. For example, Charnley obtained fixation of his acetabular components with polymethyl methacrylate in his low-friction THA. Although this type of fixation showed promising initial results, longer-term follow-up studies demonstrated that cemented acetabular components displayed concerning rates of radiographic loosening and revision.

For these reasons, late in the 20th century, focus shifted toward the development of reliable cementless fixation of acetabular components. Currently, the most commonly utilized acetabular components for primary THA are press-fit by design to achieve initial stability and coated to achieve biologic in-growth for long-term fixation. Cementless acetabular components have been shown to be associated with lower rates of loosening and revision surgery at short- and long-term follow-up when compared with their cemented counterparts.

First-generation cementless acetabular components were able to achieve biologic ingrowth but had significant failure rates associated with polyethylene wear or dissociation of the liner from the shell due to poor congruity, resulting in uneven stress distribution and unreliable locking mechanisms. Second-generation components instituted better locking mechanisms and had more congruence between the shell and polyethylene liner, minimizing complications related to modular component dissociation. However, many second-generation components had a polyethylene liner extruding outside the metal shell and a locking mechanism at the rim, which led to impingement-related complications, such as liner fracture. Third-generation components were designed to mitigate the risks of dissociation and impingement. This was achieved by eliminating protrusion of the liner beyond the rim of the acetabular shell, maintaining a high congruence between the components with smooth interfaces and recessing the locking mechanism. The third-generation designs are currently the most commonly utilized acetabular components in primary THA.

Cemented Implants

Cementation of implants in primary THA may have fallen out of favor in the United States in recent years. However, this technique has demonstrated excellent survival rates and outcomes, especially with modern-day cementing techniques. Some of the most common indications for utilizing cemented implants in THA include poor bone quality (or osteoporotic bone), irradiated bone, and Dorr C proximal femoral anatomy ^, ( Fig. 18.4 ). Multiple European joint registry studies have demonstrated a lower all-cause revision rate and decreased revision burden when comparing cemented with uncemented THA, and many surgeons still believe that cementation is the gold-standard for the elderly patient population.

Spinopelvic Relationship

In recent years, the spinopelvic relationship has become a topic of interest as it has been shown that patients with deformity, imbalance, or other conditions affecting the flexibility of their spine are at a significantly greater risk for postoperative hip instability. The spinopelvic relationship is influenced by postural changes and may alter functional acetabular component version. Morton et al. describe how stiff or imbalanced spines can alter the protective motion of increased posterior pelvic tilt, resulting in increased functional acetabular version when going from a standing to a seated position. This could significantly decrease the range of impingement-free motion and increase the risk of dislocation. With this knowledge, the arthroplasty surgeon should identify patients who have spinopelvic-related risk factors and adjust the surgical plan accordingly. An easy way to assess spinopelvic motion in the preoperative setting is to compare the anterior pelvic line and sacral slope on lateral standing and seated radiographs of the pelvis ( Fig. 18.5 ) . If there is less than 10 to 15 degrees of change in either angle, spinopelvic motion is limited. If this is identified or if a patient has an ankylosed lumbar spine or known lumbosacral spinal fusion, the surgeon may decide to utilize a dual mobility construct or place the acetabular component in a more anteverted position, accommodating for the lack of posterior pelvic tilt with postural change.

Intraoperative Considerations and Complications

Positioning of the Acetabular Component

There are many factors involved with determining proper acetabular component position. Patient anatomy, spinopelvic relations, intraoperative positioning, angular guides, surgical approach, and surgeon experience may all impact final component positioning.

Proper positioning of the acetabular component with regard to the anteversion and abduction angles is universally accepted as critical to the success and longevity of a total hip replacement. In 1978, Lewinnek et al. reported that when the acetabular component is placed in the “safe zone,” 15 ± 10 degrees of anteversion and 40 ± 10 degrees of abduction, their rates of dislocation were approximately 4 times lower than if placed outside of this zone. More recent studies have also shown that improperly positioned acetabular components can cause accelerated wear, liner loosening and/or fracture, and interprosthetic or soft-tissue impingement. ^,

At the time of surgery, patient positioning is of the utmost importance, especially when placing an acetabular component without the aid of navigation or fluoroscopy. If the patient is positioned appropriately on the operating room table, impactors with angular guides can be used to aid in proper component position ( Fig. 18.6 ). These devices range from static jigs to digital angular measuring devices. Anatomic landmarks should also be evaluated intraoperatively to assess proper component positioning. For example, the transverse acetabular ligament (TAL) can be utilized to evaluate acetabular component depth and version at the time of implantation. The inferior rim of the shell should sit just deep and parallel to the TAL if re-creation of native hip center and version is desired. Proper placement of the shell can also be assessed based on position and orientation relative to the ischium at the time of implantation. With over-medialization, the shell will sit deep to the ischial takeoff from the inferolateral aspect of the acetabulum. Anatomic landmarks can also be used to make subtle changes to native hip position and version as dictated by patient anatomy and spinopelvic parameters. For example, in patients with a spinal fusion, the surgeon may choose to dial in more anteversion relative to the TAL.

Stable Fixation of the Acetabular Component

Obtaining adequate intraoperative stability of the acetabular component during primary total hip arthroplasty is essential for achieving biological fixation and long-term implant stability. Osteointegration of the acetabular shell occurs when there is limited micromotion, <40 μm, and direct bone apposition against the component. Intraoperative instability of the acetabular shell can occur in the setting of inadequate press-fit resulting from soft-tissue interposition, under-coverage, poor bone stock or bone quality, and, less frequently, fracture of the native acetabulum.

It is not uncommon for soft tissues to become interposed between the reamed acetabulum and the acetabular shell during impaction of the final implant. If you suspect that this may be the case, it is important to increase your exposure and, if necessary, improve your retractor placement so that you have full visualization of the acetabulum without any overlying or encroaching soft tissues. If utilizing a lateral or posterior approach, gluteus maximus tenotomy should be considered to improve exposure and access to the acetabulum for unimpeded cup impaction if the femur is unable to be adequately mobilized. You may also try to use a blunt surgical instrument to sweep out the tissue from behind the shell prior to reattempting impaction.

If adequate press-fit is not achieved despite confirming lack of interposed soft tissues, it would be prudent to assess the depth of the reamed acetabulum and the amount of shell coverage at the time of impaction, as lack of implant-to-bone contact is a common reason for inadequate initial stability. If the cup appears loose due to lack of coverage, additional medial remaining without rim expansion may solve the problem. If coverage appears adequate but press fit is still not achieved, additional reaming to expand the rim may be undertaken, as bone stock allows, in order to place a larger-diameter acetabular component. Another option may be to switch to a nonhemispherical shell in order to achieve a more stable column fit between the anterosuperior and posteroinferior columns of the pelvis during impaction.

If these measures have been taken and a satisfactory press-fit is still not achieved, acetabular screws may be placed. Many surgeons choose to utilize supplemental acetabular screw fixation even in the face of adequate initial press-fit. This technique has been shown to improve initial component fixation and stability, decreasing micromotion and facilitating bone ingrowth. The risk of increasing potential joint space and the risk of neurovascular injury with screw placement must be realized and considered on a case-by-case basis.

Supplemental screw fixation may also be necessary to achieve initial stability in the setting of osteoporotic bone, if the socket is over-reamed, or if there is an intraoperative fracture of the native acetabulum. In these scenarios, multi-hole acetabular components should be used in order to achieve as many points of additional fixation as possible throughout the pelvis. This includes fixation into the ilium, ischium, and sometimes into the superior pubic ramus depending on surgeon preference and experience. In the case of fracture, a multi-hole shell may allow for internal bridging with points of fixation on either side, but if adequate stability is not achieved with this construct, supplemental plate or cage fixation may be necessary ( Fig. 18.7 ).