The Uncemented Socket

 

 

 

BACKGROUND

Cementless fixation has become the primary method of acetabular component reconstruction in North America as several reports have suggested unacceptably high failure rates of cemented acetabular fixation (1,2,3).

Contemporary acetabular implant designs incorporating surfaces favorable to biologic fixation have demonstrated excellent implant survivorship, provided that adequate initial implant stability has been obtained with interference fit or with the addition of supplemental screws or spikes (4,5,6,7,8,9,10,11). While fixation failures of contemporary cementless acetabular components have been infrequently reported, conventional ultrahigh molecular weight polyethylene (UHMWPE) wear rates have been the predominant source of failure among cementless component designs implanted during the 20th century (3,11,12,13,14,15). The introduction of highly cross-linked polyethylene into contemporary cementless acetabular components has demonstrated limited wear, even among highly active individuals, and leads to cautious optimism that this combination will result in longer acetabular component survivorship (16).

 

INDICATIONS

Cementless fixation can be utilized for all primary and revision total hip arthroplasty procedures. Diagnoses commonly addressed with primary cementless implants include the following:

Osteoarthritis secondary to acetabular dysplasia or femoroacetabular impingement Protrusio acetabuli

Osteonecrosis Inflammatory arthritis Femoral neck fracture

 

 

PREOPERATIVE PREPARATION—GENERAL CONSIDERATIONS

Attentive preoperative planning is an essential component of successful acetabular reconstruction, regardless of the fixation method utilized. The desired depth of acetabular reaming relative to the medial wall should be determined for implant fit and superolateral coverage. The placement and sizing of acetabular components may vary for hips depending on the primary diagnosis. The presence of osteophytes in the floor of the acetabulum may result in a lateralized hip center among patients with osteoarthritis and may require medialization beyond the floor of the cotyloid fossa (Fig. 13-1). Lateralization of the hip center may also occur with hip dysplasia, and the amount of medialization will vary depending on the thickness of the pelvis and the amount of preparation necessary to obtain adequate lateral implant coverage and stability (Fig. 13-2). In some cases of severe acetabular protrusio, it may be necessary to lateralize the hip center into a more normalized position (Fig. 13-3).

 

 

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FIGURE 13-1 A: Hip osteoarthritis with lateralized hip center. B: Templated.

 

 

 

 

 

FIGURE 13-2 A: Hip dysplasia with lateralized hip center. B: Templated.

 

 

 

 

 

FIGURE 13-3 A: Acetabular protrusio. B: Templated.

 

 

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Medialization of the acetabular component is important to reduce biomechanical forces on the hip and to provide adequate contact between the implant and the acetabular bone for successful osseointegration. Traditional techniques used in cemented total hip arthroplasty have advocated for acetabular component placement within 1 to 2 mm of the medial acetabular wall (17). In cases where the acetabulum has been medialized substantially relative to patient's native hip center, this may result in a decrease in tension of the gluteus medius and minimus

muscles and potentially contribute to prosthetic joint instability. Femoral offset or length may need to be increased in order to optimize hip stability in these cases. Charnley's surgical approach, utilizing a trochanteric osteotomy, facilitated advancement of the trochanter in order to improve soft tissue tension with the reconstruction. However, concerns of trochanteric nonunion have limited the use of this exposure in contemporary arthroplasty. Acceptance of a slightly more lateral acetabular component position relative to the medial wall may be preferred for cases where medialization will substantially reduce femoral offset. It may also be an acceptable consideration for younger THA patients in whom the preservation of medial and posterior bone will facilitate safe removal of well-fixed components if indicated in the future (e.g., malposition, instability, infection).

Knowledge of acetabular implant design is also important. Enlargement of the peripheral rim of the acetabular component may vary with implant manufacturer and component design. Selection of the appropriate final reamer should allow for a 1- to 2-mm press fit of the acetabular component against the prepared acetabular bone according to implant design specifications. With some of the newer porous metals, line-to-line reaming has been advocated. Surface characteristics and implant stiffness will dictate the reaming technique.

TECHNIQUE

Surgical Exposure

Several anatomical approaches to the hip are available and may be selected based on patient preferences and surgeon experience. Details of the surgical approaches have been described in Chapters 1, 2, and 3. Acetabular exposure should include placement of retractors as necessary to provide adequate visualization of the acetabulum and the transverse acetabular ligament. The acetabular labrum should be excised to allow a hemispherical reamer to engage concentrically into the acetabulum. Soft tissue should be removed from the cotyloid fossa to allow visualization of the floor of the native acetabulum. A thin-tipped, curved clamp can be useful in defining the depth of the cotyloid fossa prior to reaming (Fig. 13-4) and can be used to probe through the medial acetabular bone to define the depth before contact of the medial wall during reaming, if desired.

 

 

 

 

FIGURE 13-4 Defining the cotyloid fossa depth for reaming.

 

 

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Bone Preparation

Acetabular bone preparation includes a combination of medialization and graduated expansion in order to

recreate a hemispherical bone bed for component insertion. Initial reaming should be accomplished to establish the desired depth of reaming (Fig. 13-5). The initial direction of acetabular reaming may be influenced by the depth of the native acetabulum and the amount of medialization that is required to obtain at least 75% to 80% contact of the acetabular component against the native acetabulum (18,19). After establishing the desired depth of reaming, progressive expansion with additional reamers is continued until good contact of the reamer between the anterior and posterior columns of the native acetabulum has been achieved. Visible filling of the reamer will commonly occur when the acetabulum has been medialized (Fig. 13-6). Care should be exercised to avoid excessive reaming of the lateral wall, posterior wall, and posterior column of the acetabulum

 

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to avoid potential compromise of implant fixation. Eccentric reaming of the posterior wall can occur from a

posterior approach if the femur is not adequately translated anteriorly. Eccentric reaming of the lateral wall can occur if the acetabular reamer is not adequately seated into the acetabulum and reaming is directed proximally into the pelvis. Eccentric reaming of the posterior column can occur if the reamer is placed in a position of increased anteversion and the acetabulum is reamed to an increased depth. Assessment of the anterior, lateral, and posterior walls should be accomplished at intervals during acetabular preparation, particularly as the templated acetabular component size is approached or exceeded.

 

 

 

 

FIGURE 13-5 Initial reaming to medialize hip center towards templated position.

 

 

 

FIGURE 13-6 Progressive reaming to medialize and expand to engage the acetabular rim.

 

 

 

 

 

FIGURE 13-7 Component impacted with 40 degrees abduction and 15 to 20 degrees anteversion target.

 

The acetabular component should be positioned for impaction with a target between 40 and 45 degrees of abduction and approximately 15 and 20 degrees of anteversion (Fig. 13-7). While the primary consideration for patients positioned in a lateral position for total hip arthroplasty has focused on leveling the table and limiting patient malrotation, variability in either coronal plane alignment (pelvic tilt) or sagittal plane mobility (kyphosis/lordosis) can influence the relative position of the patient's pelvis and contribute to acetabular component malposition, which has been reported to occur in 12% to 50% of THA cases (20,21). Intraoperative imaging may be helpful to reduce errors in component alignment and should be considered in cases where component position/coverage with targeted alignment does not reflect the component position and coverage expected with preoperative templating.

Uncomplicated Primary Total Hip Arthroplasty

In the absence of significant acetabular deficiency, the authors' preferred technique of acetabular bone preparation incorporates symmetric medial and peripheral reaming with a limited number of reamers to the desired depth for acetabular coverage. The initial acetabular reamer is selected between 3 and 5 mm smaller than the templated implant size (with a 1-mm press fit target) and is positioned into the desired orientation of reaming while the reaming is carried transversely towards the floor (medial wall), rather than in a predominantly superior direction. Acetabular reaming should be continued until the floor of the cotyloid fossa has been reached. Additional reaming is performed as necessary until adequate coverage of the acetabular reamer has been attained. As long as adequate posterior and lateral acetabular bone remains, reaming is generally continued up to one size larger than the templated component size until resistance is noted between the final reamer and the native acetabulum. The acetabular component is impacted into the targeted position and stability of the construct is assessed. If adequate press fit stability is present, the surgeon may choose whether or not to supplement fixation with additional screws. The authors' preference is to use supplemental screw fixation in all acetabular reconstructions (Fig. 13-8A,B).

 

 

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FIGURE 13-8 Uncomplicated primary THA. A: Preoperative. B: Postoperative.

 

Acetabular Dysplasia

In cases of acetabular dysplasia, consideration should be given to place the acetabular component near the medial acetabular wall, to optimize biomechanical forces across the reconstruction (Fig. 13-9A,B). When a low hip center is selected, intraoperative fluoroscopy may be helpful to confirm the location of the native acetabulum. Initial preparation is initiated with a small reamer (36 to 40 mm) to the depth of the medial acetabular wall with subsequent expansion until the reamer has achieved stable contact between the anterior and posterior columns. Controlled medialization of the acetabulum 1 to 2 mm beyond the acetabular wall may be utilized in order to achieve adequate lateral implant coverage in cases with acetabular dysplasia (18). Structural lateral wall defects can be treated with either structural grafts or commercially available metal augments, but these have been uncommonly used in our practice. When using a structural graft, preparation of the lateral acetabular bone may be accomplished with a high-speed burr or acetabular reamer to match the contour of the selected graft or augment material. Structural grafts should be inserted and secured provisionally with large pins to establish stability as the final reamer engages both the host acetabulum and loaded portion of the structural graft.

Definitive fixation of the structural graft is accomplished with large cancellous screws supported by washers using a lag technique prior to placement of the selected acetabular component. In cases where structural

 

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deficiency is present, a multiholed component should be considered to increase supplemental fixation options. In some cases of acetabular dysplasia, the native acetabular component may have worn into a slightly superior position that will remain acceptable for primary component positioning. Reconstruction of the acetabulum with a slightly high hip center may be prepared in a similar fashion to a primary THA. Care should be accomplished when placing an acetabular component into a high hip location, as the anterior and posterior columns are closer together and expansion of the acetabulum will generally result in removal of bone from the anterior or posterior columns. This can contribute to potential loss of implant support and decreased availability of adequate bone for supplemental screw placement.

 

 

 

 

FIGURE 13-9 Acetabular dysplasia. A: Preoperative. B: Postoperative.

 

Acetabular Protrusio

Cementless acetabular reconstruction techniques can be utilized to address major structural deficiencies of the medial wall, as are encountered in acetabular protrusio (22,23). Acetabular reamers close to the templated implant size are utilized to prepare the peripheral acetabulum. The peripheral acetabulum is reamed until a stable contact is obtained with the acetabular reamer. Autograft from the patient's femoral head or allograft bone is prepared and impacted into the floor of the native acetabulum. The authors' technique obtains bone from femoral head autograft or allograft(s) using small acetabular reamers, placement of the milled bone into the acetabular defect, and reverse impaction using a reamer 2 mm smaller than the final selected reamer. A multiholed acetabular cup is selected, impacted into position, and stabilized with multiple screws (Fig. 13-10A,B). In rare cases with substantial superomedial bone loss, acetabular augments may be considered as an alternative support for the acetabular component. The authors' practice has not required the use of structural augments in primary THA for acetabular protrusio.

 

 

 

FIGURE 13-10 Acetabular protrusio. A: Preoperative. B: Postoperative.

 

Irradiated Bone

Improvements in cementless acetabular component porous surfaces have offered improvement in outcomes of total hip arthroplasty for patients with previously irradiated bone (24). The technique of bone preparation is the same as previously described for uncomplicated total hip arthroplasty. Multiholed acetabular components with an enhanced porous surface (e.g., porous tantalum, porous titanium) should be strongly considered for use in these cases. Implants that allow the addition of supplemental screws after burring through the acetabular shell may be effective for these procedures, but these components require the use of a cemented liner rather than engagement of a modular liner against a conventional locking mechanism.

 

 

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PEARLS AND PITFALLS

Successful Component Alignment Requires Thoughtful Engagement

Patients are not uniformly positioned on an operative table in either the supine or lateral position. Pelvic tilt can be introduced when a patient is positioned on their side, and variability in lumbopelvic sagittal plane alignment may also be present with either supine or lateral positioning. Alterations in these pelvic relationships may affect anteversion and abduction of the patient's native acetabulum and may influence inaccurate targeting of the acetabular component—even when verified using conventional alignment guides. The transverse acetabular ligament (TAL) and anterior acetabular wall can be used as secondary checks for acetabular component position, but abnormal relationships resulting from the patient's diagnosis need to be considered. If the TAL and adjacent inferior acetabular bone are readily visible after acetabular component placement, this generally suggests that the component is placed in a horizontal position. If the TAL cannot be visualized after component insertion, the acetabular component either is placed in a vertical position or is not adequately seated. When the component is flush against the patient's anterior wall, the acetabulum has been reconstructed in a similar degree of anteversion as the patient's native acetabulum.

 

The Acetabulum Should be Prepared as a Hemisphere

Excessive reaming in depth with reamers substantially smaller than the templated size can result in the creation of an elliptical acetabular bone bed rather than a hemisphere. As reaming is continued up toward the templated size, inability to reach the depth established by the initial reamer may result a dome gap. Balanced consideration should be given to accepting a dome gap ≤2 mm in cases where progressive expansion with reaming may result in excessive removal of bone from the supporting anterior and posterior columns.

Avoid Excessive Lateral Wall Reaming

Smaller incisions/less invasive surgical approaches may decrease effective visualization of the acetabulum during bone preparation. Inadequate seating of the acetabular reamer may result in eccentric reaming of the lateral acetabular wall. If this happens, medialization of the acetabulum—even with minor penetration of the medial wall—may be helpful in obtaining adequate lateral coverage.

 

Avoid Eccentric Posterior Wall Reaming

During posterior approaches to the hip, failure to adequately mobilize the femur into an anterior position can place pressure against the acetabular reamer and direct it posteriorly. Reaming from this position can result in eccentric reaming of the posterior wall. Attention should be made to ensure that the posterior wall is not excessively thinned during the process of acetabular preparation.

 

Augment Fixation with Screw Fixation

Acceptable press fit stability may be obtained with cementless components without screw fixation in the majority of cases. However, in the authors' experience, as many as 5% to 10% of patients may not attain solid press fit stability during component impaction. Host bone quality may impact the ability of a component to achieve stable mechanical stability at the time of component placement. Consideration to use supplemental fixation may also be warranted if a larger sized femoral head (≥36 mm) is used. Increased friction between the bearing surfaces may contribute to more force transmission or micromotion to the implant-bone interface. The authors' preference is to utilize 1 to 2 screws to secure a primary cup during an uncomplicated total hip arthroplasty and at least 3 screws to secure an acetabular component when there is major acetabular dysplasia or other structural deficiencies.

 

Safe Zones for Acetabular Screw Placement

The literature has previously established safe zones for drilling and placement of acetabular fixation screws based on a line running from the ASIS to the ischium. The quadrants posterior to this line have generally been considered to be safe for placement of screws. While this is a good general guideline, it is important to use good technique when placing screws into the posterior acetabulum. The posterior column is more narrow than the ilium, with a typical screw length of 15 to 25 mm. The sciatic notch is located between the posterosuperior and posteroinferior quadrants, and it may be beneficial to palpate the sciatic notch prior to placement into this portion of the posterior column.

 

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Minor malposition of components may also result in premature penetration of the medial wall (vertical cup),

lateral wall (horizontal cup), or posterior column (anteverted cup). Gradual advancement of the drill bit during screw preparation at 2 to 4 mm increments may be helpful in preventing overpenetration of the drill bit and risk for iatrogenic injury. In cases where the depth of drilling into the superolateral quadrant will not accommodate a 35- to 40-mm screw, it is important to assess the component position. Intraoperative radiographs may be helpful to define the position of the patient's pelvis and to optimize acetabular component placement.

POSTOPERATIVE MANAGEMENT

Most cases where cementless acetabular components are utilized can be managed with immediate weightbearing. For cases where the initial press fit stability of the acetabular or femoral components was not felt to be optimal or when there is greater than 15% to 20% of the acetabular implant not positioned against the host bone, the authors' preference is to limit weightbearing with the use of a walker or crutches for the initial 6 weeks after surgery. Other management considerations are consistent with those utilized for all total hip arthroplasty patients.

COMPLICATIONS

Most complications associated with the use of cementless acetabular components are typical of the total hip arthroplasty procedure and are not appreciably different from those experienced when cemented components are utilized. The midterm and long-term implant failure rates are anticipated to be lower with the use of contemporary cementless components with enhanced surfaces for osseo-integration. Concerns related to malposition of components or technical errors related to other elements of the procedure are anticipated to predominate the risks for surgical intervention, regardless of acetabular component fixation technique. Suboptimal implant position can result in increased rates of instability and increased risks for complications associated with hard-on-hard bearings, including increased rates of audible squeaking with ceramic-on-ceramic couples, increased bearing surface wear, or metal-related wear conditions.

 

 

RESULTS

Overall, cementless acetabular components have improved the survivorship for total hip arthroplasty constructs. Mechanical stability and osseointegration have become more predictable with enhanced material surfaces that facilitate osseointegration. Improvements in bearing surface locking mechanisms and polyethylene quality have contributed to the outstanding clinical performance and survivorship of contemporary bearing surfaces. Cementless components remain our implant of choice for acetabular reconstructions following both primary arthroplasty and revision total hip arthroplasty.

 

 

REFERENCES

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19. Nawabi DH, Meftah M, Nam D, et al.: Durable fixation achieved with medialized, high hip center cementless THAs for Crowe II and III dysplasia. Clin Orthop Relat Res 472: 630-636, 2014.

     

     

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24. Joglekar SB, Rose PS, Lewallen DG, et al.: Tantalum acetabular cups provide secure fixation in THA after pelvic irradiation at minimum 5-year followup. Clin Orthop Relat Res 470: 3041-3047, 2012.