Cementless acetabular revision for severe acetabular bone loss without pelvic discontinuity (type IIIA and B defects)

Introduction

Severe acetabular bone loss poses a challenging reconstructive problem in revision total hip arthroplasty (THA), with some of the most complex patterns seen in Paprosky type IIIA and IIIB defects. Even in the absence of chronic pelvic discontinuity, the treatment of IIIA and IIIB remains a significant operative challenge. In acetabular revisions, fully cementless constructs offer several advantages over cemented or hybrid components. First, there is the potential for long-term stability secondary to biologic fixation. Second, cementless fixation eliminates failure at the bone-cement and cement-implant. Furthermore, the literature supports excellent survivorship. ^, Premise: Appropriate treatment with a cementless construct requires careful consideration of implant choice, adequate surgical exposure, and a systematic approach to the management of bone loss.

Surgical approach and exposure

Appropriate surgical exposure is critical to cementless acetabular revision. Surgeon experience, prior surgical approach, and specific location of the defect may drive the decision-making on which approach is used; however, the workhorse remains the posterolateral approach. This approach offers options for extensile exposure of the acetabulum and iliac wing. Although this approach is sufficient for the vast majority of acetabular revisions, one limiting factor can be mobilization of the femur, which can be achieved through resection of anterior femoral scar tissue, or one of several described trochanteric osteotomies.

Explantation of the acetabulum and the planned reconstruction requires a systematic approach. First, adequate surgical exposure must be achieved as described above, with full visualization of the acetabulum and the surrounding defect. Once the appropriate components have been removed, excision of scar tissue and granulation debris is necessary to fully evaluate the defect and remaining bone stock and confirm the absence of a chronic pelvic discontinuity or associated fracture. Explantation of the acetabular component can be performed using a variety of techniques based on surgeon experience and preference. Explant systems with sharp curved blades, gouges, or osteotomes can be used based on surgeon preference. However, the fundamental principle remains the same: minimize the amount of bone loss and avoid traumatizing any adjacent soft tissue in cases of severe migration.

Algorithmic approach to cementless reconstruction of IIIA and IIIB defects

After component extraction and appropriate visualization, the classification of the defect should be confirmed. An algorithmic approach described by Sporer et al. is useful in developing a strategy for the management of IIIA and IIIB defects. First, it must be determined if there has been over 3 cm migration of the hip center. Second, intraoperatively, the surgeon needs to discern whether full inherent stability with a hemispherical trial component is possible. If the surgeon is able to achieve partial inherent stability, then the defect can be classified as IIIA.

Type IIIA defects can then be further classified based on the type of remodeling into a spherical shape (less common) or oblong shape (more common). Cementless reconstruction for spherical-shaped IIIA defects is best served by large hemispherical cups (jumbo cups). In contrast, in the more common oblong-shaped type IIIA defects, there are several reconstruction options depending on the surgeon’s approach to the hip center. To restore the hip center in IIIA defects, cementless options include hemispherical cup reconstruction with modular porous metal augments, structural graft (i.e., bulk distal femoral allograft) with a hemispherical cup placed at the anatomic hip center, oblong/bilobed components, or the more recently described double cup technique. If the surgeon does not wish to recreate the anatomic hip center, then the surgeon can attempt to achieve stability with a hemispherical cup placed in the position of a high hip center.

There are several advantages of recreating the native hip center, which include restoring the natural center of rotation and decreasing the joint reactive force, which has advantages in terms of decreased loosening rates and improved wear characteristics. To recreate the native hip center, the superior aspect of the defect must be filled by either a metal component (metal augment or secondary cup) or allograft, which allows for partial initial stability of the anatomically placed hemispherical cup, with the ultimate goal of biologic fixation with bony ingrowth. One important caveat is that in cases of chronic proximal migration, where the periarticular soft tissues have contracted, there could be scarring around the sciatic and/or femoral nerve. In these circumstances, a high hip center may be preferred to prevent neurologic stretch injury, or the surgeon may need to consider a subtrochanteric shortening osteotomy. A high hip center approach may also allow for maximum preservation of the remaining host bone.

If the surgeon determines that there is no inherent stability of a hemispherical socket, the defect can be classified as type IIIB. In this circumstance, the algorithm divides into non-biologic or biologic fixation strategies. Non-biologic fixation strategies include structural allograft (i.e., bulk femoral or acetabular) or impaction bone grafting with a supporting cage construct, but these typically require cementation as part of their strategy – this will be discussed in Chapter 9 . Cementless biologic fixation strategies in the absence of discontinuity also include metal augments and custom tri-flanged components.

Large hemispherical cups without augmentation (Jumbo cups)

In type IIIA defects that are spherical in nature, initial stability between the anterosuperior and posteroinferior columns can be achieved with the use of a large hemispherical cup alone, also known as a jumbo cup. When appropriately indicated, the advantage of this technique is its relative technical simplicity, ability to use a standard hip exposure, reduction in trauma to abductors, and ability to maintain a near-anatomic hip center. In addition, with the increased contact area between the host bone and porous cup, there is sufficient friction to achieve immediate initial stability as with a press-fit component that obtains long-term biologic stability ( Fig. 8.1 A and B). In addition, because of their large size, jumbo cups also allow for sufficient polyethylene thickness to accommodate a large femoral head, in which maximizing the component’s contribution to hip joint stability becomes critical given potential soft tissue compromise.

Following adequate exposure, prior component removal, and soft tissue debridement, sequential reaming can begin. The surgeon should attempt to maintain the native hip center with this technique. Careful attention must be paid to ensure that excessive reaming of the medial wall, residual roof, and posterior wall are avoided. Unlike in the primary setting where bone is removed, reaming in revision surgery is more bone contouring to remove high spots and level the surface—very little bone is removed.

Technically, we recommend identifying the inferior margin of the acetabulum prior to reaming. Once the inferior margin of the acetabulum is identified, one can use that landmark to estimate the position of the native hip center. The reamer is then used to contour bone and gradually enlarge the socket until there is contact between the anterosuperior and posteroinferior columns. Depending on available bone stock, the socket may be medialized slightly. This technique minimizes the chance of inadvertent removal of the superior bone stock.

Once sequential reaming is complete, the surgeon then assesses socket stability using either a trial implant or the reamers themselves. If the surgeon determines the socket is inherently stable, a cementless cup can be placed. The degree of press-fit is dictated by the component design (hemispherical or elliptical) and the type of fixation surface. High friction surfaces like 3D printed titanium or tantalum are usually implanted with less reamer-implant mismatch than other fixation surfaces like titanium fiber-metal mesh. It has been stated that more than 60% bony contact is necessary for stability, and 5-10 mm of un-coverage may be acceptable. Although surgeons can use this as a rule of thumb, several revisions have been performed with less host bone contact and larger surface areas of un-coverage when using large porous-coated acetabular components—the key is implant stability. Several multidirectional screws that allow for stabilization of the bony pelvis are recommended. The goal is to provide rigid implant fixation to allow bone ingrowth to occur ( Fig. 8.1 A and B).

Although jumbo cups offer many advantages, they are not without limitations. Ultimately, sequential reaming, even in hemispherical defects in which the superoinferior diameter of the acetabulum is larger than the anterior-posterior diameter, may lead to compromise of the residual columns. If this is noted intraoperatively, adjustment of the surgical technique to either a high hip center or reconstruction with augmentation (metal or allograft) should be considered. The surgeon should be cautioned to make this determination early, given that a jumbo cup approach is not bone-preserving, unlike the other described reconstruction techniques.

The results of jumbo cups are largely favorable in IIIA defects. Although many of the series do not perform a sub-group analysis for IIIA defects alone, overall survivorship for jumbo cup constructs is favorable in several reported series. Gustke et al. found in a series of 196 jumbo cups (of which 17% of patients had IIIA defects and 8% had type IIIB defects) with an average of 10 years of follow-up, overall survivorship of 98% at four years and 96% at 16 years. Lachiewicz et al. studied 129 jumbo cups and found, with failure defined as cup removal for any reason, ten-year survivorship of 94% and 15-year survivorship of 80%. Although survivorship by Paprosky type was not looked at specifically, the authors did not find an association between Paprosky type and differences in the failure rate.