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Commentary & Perspective | ||||||||
Commentary & Perspective on Udomkiat and colleagues reviewed one-hundred and thirty-two selected (not consecutive) primary total hip replacements performed over a thirty-month period with use of a porous-coated acetabular component implanted with a press-fit technique without screws. One hundred and three patients (110 hips) were available for a minimum ten-year clinical and radiographic follow-up. One (0.9%) of 110 hips was revised because of aseptic loosening of the acetabular metal shell at eighty-four months postoperatively. Survivorship of the acetabular component was 99.1% at twelve years with revision or recommended revision for aseptic loosening as the end point and 95.3% with revision of the metal shell for any reason as the end point. The most common reason for reoperation in this series was wear or disassociation of the polyethylene liner. The strengths of this study include the large number of patients that were operated on by a single surgeon, the long (10.2 years) average duration of follow-up, and the use of both radiographic and clinical criteria in the analysis of the results. The primary limitation of the study is its ambiguity regarding the selection criteria that determined which patients received augmentation of fixation with screws for primary fixation of the acetabular component. The authors mentioned in the Discussion that, in their estimation, "some sockets will always require screw fixation". These include those in patients with soft bone secondary to osteoporosis, a dysplastic socket with an absent anterior wall leaving the superior aspect of the cup uncovered by bone, and cups that were not entirely stable with press-fit implantation alone. In the Materials and Methods section, the authors stated that intraoperative stability of fixation was confirmed by checking for movement after hitting the socket edge with a metal bone tamp and mallet and by trying to pull the metal shell out of the acetabulum with use of the attached insertion tool (defined as a "pull-out test"). We question the accuracy and reproducibility of such tests for determining the intraoperative stability of cup fixation. Orthopaedic surgeons have long debated the issue of whether to use supplemental screw fixation when implanting cementless acetabular components. Proponents of screw fixation cite better initial stability and increased bone ingrowth around holes with screw fixation as advantages1,2. Opponents of screw fixation point to the risk of vascular injury, the potential for fretting corrosion at the interface of the screw and the cup, and the potential conduit afforded by screw holes for polyethylene wear debris (leading to retroacetabular osteolysis)3. Several clinical studies have examined the results of cementless acetabular components inserted both with and without screws. Clohisy and Harris4 reported the clinical and radiographic results at an average ten-year follow-up of 237 consecutive primary acetabular reconstructions that were performed with use of a Harris-Galante porous-coated acetabular component (HGP; Zimmer, Warsaw, IN) with screw fixation. They found no complications associated with the insertion of acetabular screws; no acetabular component migrated or was classified as radiographically loose; and none was revised because of aseptic loosening. However, eight (4%) of 196 acetabular shells were revised, including two because of retroacetabular osteolysis. Tompkins et al. reported the results at an average nine-year follow-up of 173 primary total hip arthroplasties that were performed at our institution with insertion of a Harris-Galante type-I acetabular component without cement5. A line-to-line technique was used to insert the acetabular component, and three to six cancellous dome screws were placed in each component for supplemental fixation. Although osteolysis was seen in 4% of the acetabula, survivorship analysis at ten years revealed that the acetabular component had a 99% chance of survival with revision or aseptic loosening as the end point and a 97% chance of survival with revision, aseptic loosening, or reoperation because of a problem related to the acetabular component as the end point. The most common reason for reoperation in our series was excessive polyethylene wear and/or retroacetabular osteolysis. Schmalzried and colleagues reported a retrospective review of 122 primary total hip arthroplasties in which the Harris-Galante porous ingrowth acetabular component had been inserted with use of a press-fit technique without screw fixation3. They found no acetabular fractures, no evidence of radiographic or clinical loosening, no disruption of the titanium porous mesh, and no acetabular osteolysis at an average follow-up of fifty-six months. Retrieval analysis has revealed important histological findings in regard to bone ingrowth with use of cementless acetabular components. Cook et al. performed histological analyses of forty-two uncemented, porous-coated acetabular components that were retrieved at an average of 14.5 months after implantation6. Bone ingrowth was reported in twenty eight (67%) of the acetabular cups, but only 38% had ingrowth into 2% or more of the available pore volume. Interestingly, they found that bone ingrowth occurred more frequently, in greater amounts, and was more evenly distributed anatomically in cups inserted with use of supplemental screws for initial fixation. In an autopsy-retrieval study, Pidhorz and colleagues performed a histological analysis of eleven cementless porous-coated acetabular components that had been in situ for an average of forty-one months1. They reported an average bone ingrowth volume fraction of 12%, with more bone adjacent to screw holes through which screws had been inserted in comparison with the amount of bone adjacent to empty screw holes. Many factors other than screw fixation play a role in determining the success of cementless acetabular fixation. Among these are the design of the cup (hemispheric versus non-hemispheric), the surface area available for ingrowth (e.g., cobalt-chromium beads versus titanium mesh), the design of the reamer, the surgical technique, and the quality of bone stock. Some surgeons advocate line-to-line reaming of the acetabulum with the use of screw fixation. Proponents of acetabular fixation without screws usually advocate under-reaming the acetabulum by a range of 1 to 4 mm in most primary total hip replacements, depending on the quality of the bone and the need for supplemental bone graft. Results of cadaver studies leave questions about whether or how much to under-ream and about the trade-off between the benefits of achieving increased torsional stability with use of a press-fit technique and the risks of periprosthetic fracture and/or incomplete seating of the prosthesis2,7. We recommend use of adjunctive screw fixation in patients with osteopenic bone, rheumatoid arthritis, or moderate to severe dysplasia, and whenever bone graft (either morselized or structural) is required. We currently use a cup that has a cluster of holes at the apex, to allow for adequate screw fixation while limiting the number of empty holes available for the passage of retroacetabular wear debris. One potential concern in advocating the universal use of fixation without screws is that surgeons inexperienced in screw fixation techniques who do encounter a case in which screw fixation is necessary will not be able to achieve optimum insertion of the component. This well-written paper by Udomkiat et al. adds to the growing body of literature on the outcome of cementless acetabular components in primary total hip replacement. Although it provides important information regarding the use of cementless porous-coated cups inserted without screws, it leaves several important questions unanswered. Future clinical and laboratory studies will be needed to determine the specific indications for the use of screws, whether that determination can be made preoperatively (and therefore allow implantation of a cup with no holes), and how to best assess the intraoperative stability of a cementless cup. Until these important questions are answered, the debate regarding the use of supplemental screw fixation with cementless acetabular components will continue. *In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Zimmer, Inc. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer, Inc). Also, a commercial entity (Zimmer, Inc.) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated. References 1. Pidhorz LE, Urban RM, Jacobs JJ, Sumner DR, Galante JO. A quantitative study of bone and soft tissues in cementless porous-coated acetabular components retrieved at autopsy. J Arthroplasty. 1993;8:213-25. | ||||||||
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