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A retrospective clinical and radiographic review of 173 hydroxyapatite-coated screw cups with 5- to 10-year follow-up, showing low revision rates for fixation failure
The Journal of Arthroplasty Vol. 17 No. 7 2002
A Retrospective Clinical and Radiographic Review of 173 Hydroxyapatite-Coated Screw Cups With 5- to 10-Year Follow-Up, Showing Low Revision Rates for Fixation Failure C. J. Mann, FRCS (Orth), S. McNally, FRCS (Orth), E. Taylor, FRCS, and J. A. N. Shepperd, FRCS
Abstract: We reviewed the midterm results of 173 hydroxyapatite-coated screw cups. The average follow-up was 6.5 years (range, 5–9 years). The follow-up rate was 93%. Patients were assessed using the Merle D’Aubigne–Postel clinical scoring scale and by radiographic review. Two patients had revision surgery for recurrent dislocation (1.2%), 3 patients were revised for aseptic loosening (1.7%), 1 patient underwent revision surgery because of deep prosthetic infection (0.6%), and 2 patients were revised for polyethylene wear without loosening (1.2%), which gave a total revision rate of 4.7%. The average postoperative Merle D’Aubigne–Postel scores were 5.7 for pain, 5.5 for range of motion, and 5.4 for function. Key words: total hip arthroplasty (THA), screw cups, hydroxyapatite, ceramic head, acetabular component. Copyright 2002, Elsevier Science (USA). All rights reserved.
Reports of the results of uncoated screw-cup arthroplasties have been disappointing with high revision rates [1–3]. A more recent report showed an acetabular revision rate of 5.3% for aseptic loosening but with a much higher revision rate overall, with revision done mainly for excessive polyethylene wear [4]. The long-term results of the femoral component of total hip arthroplasty (THA) improved largely as a result of third-generation and fourthgeneration techniques [5] and the introduction of hydroxyapatite (HA)-coated components [6]. Atten-
tion subsequently turned to the acetabular component. The long-term results of cemented acetabular components of THA are a problem, particularly in young patients. The loosening rate of acetabular components begins slowly, then rises exponentially after 5 years [7]. Attempts to improve the survival of the cemented acetabular component by improving cementing technique in the same manner as femoral components have not proved successful [8] to date. The uncemented screw cup was developed to increase the surface area of the prosthesis available to host– bone for bonding. In addition, by using the screw thread, the strong initial fixation should help to prevent micromotion, one factor known to predispose to aseptic loosening [9]. In general, however, the results of screw cup prostheses have not been good. The Mecring prosthesis (Mecron, Berlin, Germany) had a titanium threaded ring and was used extensively in the United States and Europe
From the Conquest Hospital, The Ridge, St. Leonards-on-Sea, East Sussex, United Kingdom. Submitted August 17, 2001; accepted April 10, 2002. No benefits or funds were received in support of this study. Reprint requests: C. J. Mann, FRCS (Orth), Norfolk and Norwich University Hospital, Colney Lane, Norwich NR4 7UY UK. E-mail: [email protected]. Copyright 2002, Elsevier Science (USA). All rights reserved. 0883-5403/02/1707-0027$35.00/0 doi:10.1054/arth.2002.34825
851
852 The Journal of Arthroplasty Vol. 17 No. 7 October 2002 and had poor long-term results [10,11]. The high failure rate was due to excessive contact pressure secondary to its sharp threads and failure to achieve microinterlock [11]. Uncemented screw cups covered by HA have been encouraging, however, with excellent survival rates (99% cup survival at 6 years) as reported by Geesink and Hoefnagels [12]. The purpose of this study was to evaluate the midterm results of a HA-coated screw cup prosthesis.
Materials and Methods From February 1989 to July 1993, 173 HAcoated screw cups (Joint Replacement Instruments, London, UK) were inserted by the senior author (J.A.N.S.) or by senior registrars under supervision who were trained to use the same technique. The approach used was the anterolateral approach of Watson-Jones in all cases. All patients received a ceramic head of 32-mm diameter to articulate with the HA-coated metal-backed polyethylene liner. All femoral components were cementless, collarless, and fully HA-coated (Joint Replacement Instruments, London, UK). Every patient received subcutaneous heparin postoperatively while an inpatient. All patients had follow-up examinations after discharge at 6 weeks, 3 months, 6 months, 1 year, and annually thereafter. At each office visit, the patient was assessed clinically and radiologically. The clinical assessment was based on the Merle D’Aubingne–Postel (MDP) scoring system [13]. Each patient had a standard anteroposterior and lateral radiograph of the hip. Radiographs were available for 142 of the 173 THAs (83%) at the time of the review when 138 patients of the original 150 were available for follow-up (for further explanation, see subsequently). The radiographs were examined for radiolucent lines in all zones of DeLee and Charnley [14]. In addition, the presence of excessive polyethylene wear, uncovering of the cup superolaterally, and prosthetic migration was sought. We distinguished between osteolysis and radiolucent lines because we believe radiolucent lines are an adaptive phenomenon as described by Freeman [15], and there is a difference between a stable radiolucent line and ballooning osteolysis. A radiolucent line was considered significant if it was ⱖ2mm in diameter in any zone. If a radiolucent line of any size extended through all 3 zones, this was considered a sign of radiographic loosening. There were 150 patients with 173 screw cup prostheses, which included 23 bilateral THAs, of which 8 were bilateral and simultaneous. There were 57 men and 93 women. The average age at
index operation was 69 years, 10 months. At the time of the last review of this group of patients (1998), 33 had died, all from causes unrelated to their original THA. These patients had similar clinical scoring to the living patients at their last visit, and these were included in the average scores as described subsequently. Including the number of patients who have died, the average follow-up was 6.5 years (range, 5–10 years). From the original 150 patients, 138 were followed, giving a follow-up rate of 92%. The reason that 100% was not achieved was difficulty in tracing patients if they had moved and subsequently changed their general practitioner (8 patients) or they had refused follow-up (4 patients). A similar follow-up rate was scored for the number of hips (93%) because none of the patients who were lost to follow-up had bilateral THAs. Some patients were too infirm to be reviewed in the hospital, in which case they were visited at home (5 patients with 5 screw cup arthroplasties). A clinical assessment was made during a home visit, and if the patient was asymptomatic, no further radiographs were ordered. If the patient had significant pain or an abnormality on the previous radiograph, further radiographic studies were obtained.
Results Clinical Scoring We scored the patient according to the MDP system. The results show that clinically most patients were good or excellent with high average scores (Table 1). Radiographic Review The radiographs were reviewed to look for radiolucent lines, migration, polyethylene wear, and coverage. Although a radiolucent line of 1 mm was recorded, we did not think this represented a significantly loose cup in its own right, particularly because it may represent an adaptive phenomenon rather than true loosening. We defined a radiographically loose cup as one with a radiolucent line of ⱖ2 mm around the entire circumference of the
Table 1. Clinical Review Average MDP score for pain Average MDP score for movement Average MDP score for function
5.4 (range, 1–6) 5.3 (range, 2–6) 5.0 (range, 1–6)
Hydroxyapatite-Coated Screw Cups
prosthesis (Fig. 1, Table 2). We did not specifically look for the number of cups with lucencies in 2 zones because any patient with any radiolucent line (ⱖ1) was followed up at least annually. We looked for osteolysis, particularly ballooning osteolysis because this may indicate excessive polyethylene wear (Fig. 2). There was 1 patient with osteolysis in zone 1 and 1 patient with osteolysis in zone 3. One patient had evidence of osteolysis in all 3 zones. Two patients had osteolysis in zone 2.
• Mann et al.
853
Table 2. Radiolucent Lines Zone
Thickness (mm)
No. Patients
1
1 2 1 2 1 2 1 or 2
11 3 2 0 8 2 2
2 3 All 3 zones
Revisions A total of 8 revisions have been done from the original 173 prostheses (4.7%). The underlying causes for the revisions were as follows: infection, 1 (0.6%); augmentation for dislocation, 2 (1.2%); polyethylene wear without loosening, 3 (1.7%); and aseptic loosening, 2 (1.2%). At the time of this writing (2001), another patient has undergone a revision for excessive polyethylene wear, and this revision has undergone a later manipulation under anesthetic for dislocation. This patient has been included in the aforementioned figures. Two patients underwent revision for polyethylene wear, both of whom had stable metal backs. One patient underwent simple liner exchange, and the other patient underwent revision of the metal back because of damage to the metal back by the prosthetic femoral head—mode 4 wear [16]. Despite this wear, the HA-coated metal back remained firmly fixed and was not easy to remove.
There are a further 5 cups that meet our criteria for radiographic loosening—a radiolucent line of ⱖ2 mm over all 3 zones of Charnley and DeLee. If these cups are included, the most pessimistic revision rate for aseptic loosening is 4.1%.
Fig. 1. A 2-mm radiolucent line around entire prosthesis.
Fig. 2. Osteolysis in zone 2.
Polyethylene Wear Review of the radiographs revealed that 7 cups from the 142 radiographs available showed signs of significant radiographic wear (5%) on visual examination. These patients are asymptomatic, however, and there are no signs of actual or impending loosening. Migration Four cups had migrated. Of these 4 cups, 3 showed signs of polyethylene wear. In all cases, the migration subsequently had ceased and no patient had undergone revision for migration alone.
854 The Journal of Arthroplasty Vol. 17 No. 7 October 2002
Discussion The long-term problems affecting the longevity of THAs today include aseptic loosening and polyethylene wear, and these phenomena generally are believed to be linked [16 –26]. We believe that the use of HA, which is osteoinductive and osteoconductive, provides an optimal environment for bonding between host– bone and prosthesis. This bond is such that even in the presence of significant wear debris, the prosthesis does not always become significantly loose (as shown by the low revision rates for aseptic loosening). This bond offers a significant advantage over cemented cups in which revision of the polyethylene insert for wear necessitates revision of the entire prosthesis. It has been suggested by other authors that polyethylene wear may be more problematic in uncemented than cemented cups because owing to the metal back, contact stresses in the polyethylene are higher [27,28], leading to higher wear rates. This problem may be exacerbated in screw cups because difficulty in medializing and inferioralizing the prosthesis would lead to increased shear strains in the polyethylene, adding to the potential for adhesive wear. Excessive polyethylene wear has been described in uncemented screw cups, but we believe that this may have been due to the use of a large proportion (83 of 94) of stainless steel femoral heads [29]. Polyethylene wear remains an issue in uncemented joint arthroplasty, and thin polyethylene liners generally should be avoided. HA-coated acetabular prostheses or femoral prostheses may add to the overall wear debris by contributing thirdbody wear particles. Distinguishing the relative contributions of the third-body component from the inevitable wear from any bearing surface is problematic. Although 4 screw cups had not been medialized adequately, which led to leg-length discrepancy, this was a clinical problem in only 1 of these patients. In all cases, the discrepancy was ⬍1 inch. Radiolucent lines were 7 times more frequent in zone 1 than zone 2. They were 5 times more frequent in zone 3 than zone 2, which is in agreement with other authors [30]. Osteolysis was not common so that any inference in regard to zonal distribution is speculative, although it was most frequent in zone 2.
Conclusion From this midterm follow-up study, we conclude that HA may be beneficial. If screw cups are to be
Fig. 3. Poor anterosuperior coverage.
used, HA coating is mandatory. It would seem that the low level of polyethylene wear seen in this series is likely to be due to the use of a ceramic head. The ceramic/polyethylene couple is known to produce a low penetration rate compared with steel/polyethylene or cobalt-chrome/polyethylene couples [31]. For this reason, we advocate the use of a ceramic head, particularly in young people and especially when uncemented acetabular components are considered. At our institution, however, we no longer use a screw cup despite the low revision rates, owing to perioperative difficulties in achieving the correct offset and obtaining full anterosuperior prosthetic coverage (Fig. 3). If these technical difficulties were addressed, we would be more confident in recommending a HA-coated screw cup prosthesis.
References 1. Capello WN, Colyer RA, Kernek CB, et al: Failure of the Mecron screw-in ring. J Bone Joint Surg Br 75:835, 1993 2. Savilahti S, Myllyneva I, Lindholm TS, et al: Clinical outcome and survival of Link RS total hip prosthesis. J Bone Joint Surg Br 77:369, 1995 3. Simank H-G, Brocai DRC, Reiser D, et al: Middleterm results of threaded acetabular cups. J Bone Joint Surg Br 79:366, 1997 4. Rokkum M, Brandt M, Bye K, et al: Polyethylene wear, osteolysis and acetabular loosening with an HA-coated prosthesis. J Bone Joint Surg Br 81:582, 1999 5. Barrack RL, Mulroy RD, Harris WH: Improved cementing techniques and femoral component loosening in young patients with hip arthroplasty. J Bone Joint Surg Br 74:385, 1992
Hydroxyapatite-Coated Screw Cups 6. McNally SA, Shepperd JAN, Mann CV, Walczak JP: The results at nine to twelve years of the use of a hydroxyapatite-coated femoral stem. J Bone Joint Surg Br 82:378, 2000 7. Callaghan JJ, Forest EE, Olejniczak JP, et al: Charnley total hip replacement in patients less than 50 years old. J Bone Joint Surg Br 80:704, 1998 8. Dorr LD: Fixation of the acetabular component: the case for cementless bone ingrowth modular sockets. J Arthroplasty 11:3, 1996 9. D’Antonio JA, Capello WN, Jaffe WL: Hydroxlyapatite-coated hip implants: mulitcentre three-year clinical and roetgenographic results. Clin Orthop 285: 102, 1992 10. Capello WN, Colyer RA, Kernek CB, et al: Failure of the Mecron screw-in ring. J Bone Joint Surg Br 75:835, 1993 11. Harris WH: The first 32 years of total hip arthroplasty: one surgeon’s perspective. Clin Orthop 274:6, 1992 12. Geesink RGT, Hoefnagels NHM: Six year results of hydroxyapatite-coated total hip replacement. J Bone Joint Surg Br 77:534, 1995 13. Merle d’Aubigne´ R, Postel M: Functional results of hip arthroplasty with acrylic prosthesis. J Bone Joint Surg Am 36:451, 1954 14. DeLee JG, Charnley J: Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 121:20, 1976 15. Freeman MAR: Radiolucent lines: a question of nomenclature (editorial). J Arthroplasty 14:1, 1999 16. Schmalzried TP, Callaghan JJ: Wear in total hip and knee replacements. J Bone Joint Surg Am 81:115, 1999 17. Morscher EW: Current status of acetabular fixation in primary total hip arthroplasty. Clin Orthop 274: 172, 1992 18. Devane PA, Robinson EJ, Bourne RB, Rorabeck CH: Measurement of polyethylene wear in acetabular components inserted with and without cement. J Bone Joint Surg Am 79:682, 1997 19. Shih C-H, Lee P-C, Chen J-H, et al: Measurement of polyethylene wear in cementless total hip arthroplasty. J Bone Joint Surg Br 79:361, 1997 20. Schmalzried TP, Kwong LM, Jasty M, et al: The
21.
22.
23.
24.
25.
26.
27.
28.
29.
30. 31.
• Mann et al.
855
mechanism of loosening of cemented acetabular components in total hip arthroplasty. Clin Orthop 274:60, 1992 Kobayashi A, Freeman MAR, Bonfield W, et al: Number of polyethylene particles and osteolysis in total joint replacement. J Bone Joint Surg Br 79:844, 1997 Livingstone BJ, Chmell MJ, Spector M, Poss R: Complications of total hip arthroplasty associated with the use of an acetabular component with a Hylamer liner. J Bone Joint Surg Am 79:1520, 1997 Learmonth ID, Heywood AW, Kaye J, Dall D: Radiological loosening after cemented hip replacement for juvenile chronic arthritis. J Bone Joint Surg Br 71: 209, 1989 Dorr LD, Bloebaum R, Emmanual J, Meldrum R: Histologic, biochemical and ion analysis of tissue and fluids retrieved during total hip arthroplasty. Clin Orthop 261:82, 1990 Howie DW: Tissue response in relation to type of wear particles around failed hip arthroplasties. J Arthroplasty 5:337, 1990 Jasty M, Bragdon C, Jiranek W, et al: Etiology of osteolysis around porous-coated cementless total hip arthroplasties. Clin Orthop 308:111, 1994 Devane PA, Robinson EJ, Bourne RB, Rorabeck CH: Measurement of polyethylene wear in acetabular components inserted with and without cement. J Bone Joint Surg Am 79:682, 1997 Hernandez JR, Keating EM, Faris PM, et al: Polyethylene wear in uncemented acetabular components. J Bone Joint Surg Br 76:263, 1994 Rokkum M, Brandt M, Bye K, et al: Polyethylene wear, osteolysis and acetabular loosening with an HA-coated prosthesis. J Bone Joint Surg Br 81:582, 1999 Harris WH: The lessons of acetabular component fixation. Orthopaedics 18:811, 1995 Wroblewski BM, Siney PD, Dowson D, Collins SN: Prospective clinical and joint simulator studies of a new total hip arthroplasty using alumina ceramic heads and cross-linked polyethylene cups. J Bone Joint Surg Br 78:280, 1996
A Retrospective Clinical and Radiographic Review of 173 Hydroxyapatite-Coated Screw Cups With 5- to 10-Year Follow-Up, Showing Low Revision Rates for Fixation Failure C. J. Mann, FRCS (Orth), S. McNally, FRCS (Orth), E. Taylor, FRCS, and J. A. N. Shepperd, FRCS
Abstract: We reviewed the midterm results of 173 hydroxyapatite-coated screw cups. The average follow-up was 6.5 years (range, 5–9 years). The follow-up rate was 93%. Patients were assessed using the Merle D’Aubigne–Postel clinical scoring scale and by radiographic review. Two patients had revision surgery for recurrent dislocation (1.2%), 3 patients were revised for aseptic loosening (1.7%), 1 patient underwent revision surgery because of deep prosthetic infection (0.6%), and 2 patients were revised for polyethylene wear without loosening (1.2%), which gave a total revision rate of 4.7%. The average postoperative Merle D’Aubigne–Postel scores were 5.7 for pain, 5.5 for range of motion, and 5.4 for function. Key words: total hip arthroplasty (THA), screw cups, hydroxyapatite, ceramic head, acetabular component. Copyright 2002, Elsevier Science (USA). All rights reserved.
Reports of the results of uncoated screw-cup arthroplasties have been disappointing with high revision rates [1–3]. A more recent report showed an acetabular revision rate of 5.3% for aseptic loosening but with a much higher revision rate overall, with revision done mainly for excessive polyethylene wear [4]. The long-term results of the femoral component of total hip arthroplasty (THA) improved largely as a result of third-generation and fourthgeneration techniques [5] and the introduction of hydroxyapatite (HA)-coated components [6]. Atten-
tion subsequently turned to the acetabular component. The long-term results of cemented acetabular components of THA are a problem, particularly in young patients. The loosening rate of acetabular components begins slowly, then rises exponentially after 5 years [7]. Attempts to improve the survival of the cemented acetabular component by improving cementing technique in the same manner as femoral components have not proved successful [8] to date. The uncemented screw cup was developed to increase the surface area of the prosthesis available to host– bone for bonding. In addition, by using the screw thread, the strong initial fixation should help to prevent micromotion, one factor known to predispose to aseptic loosening [9]. In general, however, the results of screw cup prostheses have not been good. The Mecring prosthesis (Mecron, Berlin, Germany) had a titanium threaded ring and was used extensively in the United States and Europe
From the Conquest Hospital, The Ridge, St. Leonards-on-Sea, East Sussex, United Kingdom. Submitted August 17, 2001; accepted April 10, 2002. No benefits or funds were received in support of this study. Reprint requests: C. J. Mann, FRCS (Orth), Norfolk and Norwich University Hospital, Colney Lane, Norwich NR4 7UY UK. E-mail: [email protected]. Copyright 2002, Elsevier Science (USA). All rights reserved. 0883-5403/02/1707-0027$35.00/0 doi:10.1054/arth.2002.34825
851
852 The Journal of Arthroplasty Vol. 17 No. 7 October 2002 and had poor long-term results [10,11]. The high failure rate was due to excessive contact pressure secondary to its sharp threads and failure to achieve microinterlock [11]. Uncemented screw cups covered by HA have been encouraging, however, with excellent survival rates (99% cup survival at 6 years) as reported by Geesink and Hoefnagels [12]. The purpose of this study was to evaluate the midterm results of a HA-coated screw cup prosthesis.
Materials and Methods From February 1989 to July 1993, 173 HAcoated screw cups (Joint Replacement Instruments, London, UK) were inserted by the senior author (J.A.N.S.) or by senior registrars under supervision who were trained to use the same technique. The approach used was the anterolateral approach of Watson-Jones in all cases. All patients received a ceramic head of 32-mm diameter to articulate with the HA-coated metal-backed polyethylene liner. All femoral components were cementless, collarless, and fully HA-coated (Joint Replacement Instruments, London, UK). Every patient received subcutaneous heparin postoperatively while an inpatient. All patients had follow-up examinations after discharge at 6 weeks, 3 months, 6 months, 1 year, and annually thereafter. At each office visit, the patient was assessed clinically and radiologically. The clinical assessment was based on the Merle D’Aubingne–Postel (MDP) scoring system [13]. Each patient had a standard anteroposterior and lateral radiograph of the hip. Radiographs were available for 142 of the 173 THAs (83%) at the time of the review when 138 patients of the original 150 were available for follow-up (for further explanation, see subsequently). The radiographs were examined for radiolucent lines in all zones of DeLee and Charnley [14]. In addition, the presence of excessive polyethylene wear, uncovering of the cup superolaterally, and prosthetic migration was sought. We distinguished between osteolysis and radiolucent lines because we believe radiolucent lines are an adaptive phenomenon as described by Freeman [15], and there is a difference between a stable radiolucent line and ballooning osteolysis. A radiolucent line was considered significant if it was ⱖ2mm in diameter in any zone. If a radiolucent line of any size extended through all 3 zones, this was considered a sign of radiographic loosening. There were 150 patients with 173 screw cup prostheses, which included 23 bilateral THAs, of which 8 were bilateral and simultaneous. There were 57 men and 93 women. The average age at
index operation was 69 years, 10 months. At the time of the last review of this group of patients (1998), 33 had died, all from causes unrelated to their original THA. These patients had similar clinical scoring to the living patients at their last visit, and these were included in the average scores as described subsequently. Including the number of patients who have died, the average follow-up was 6.5 years (range, 5–10 years). From the original 150 patients, 138 were followed, giving a follow-up rate of 92%. The reason that 100% was not achieved was difficulty in tracing patients if they had moved and subsequently changed their general practitioner (8 patients) or they had refused follow-up (4 patients). A similar follow-up rate was scored for the number of hips (93%) because none of the patients who were lost to follow-up had bilateral THAs. Some patients were too infirm to be reviewed in the hospital, in which case they were visited at home (5 patients with 5 screw cup arthroplasties). A clinical assessment was made during a home visit, and if the patient was asymptomatic, no further radiographs were ordered. If the patient had significant pain or an abnormality on the previous radiograph, further radiographic studies were obtained.
Results Clinical Scoring We scored the patient according to the MDP system. The results show that clinically most patients were good or excellent with high average scores (Table 1). Radiographic Review The radiographs were reviewed to look for radiolucent lines, migration, polyethylene wear, and coverage. Although a radiolucent line of 1 mm was recorded, we did not think this represented a significantly loose cup in its own right, particularly because it may represent an adaptive phenomenon rather than true loosening. We defined a radiographically loose cup as one with a radiolucent line of ⱖ2 mm around the entire circumference of the
Table 1. Clinical Review Average MDP score for pain Average MDP score for movement Average MDP score for function
5.4 (range, 1–6) 5.3 (range, 2–6) 5.0 (range, 1–6)
Hydroxyapatite-Coated Screw Cups
prosthesis (Fig. 1, Table 2). We did not specifically look for the number of cups with lucencies in 2 zones because any patient with any radiolucent line (ⱖ1) was followed up at least annually. We looked for osteolysis, particularly ballooning osteolysis because this may indicate excessive polyethylene wear (Fig. 2). There was 1 patient with osteolysis in zone 1 and 1 patient with osteolysis in zone 3. One patient had evidence of osteolysis in all 3 zones. Two patients had osteolysis in zone 2.
• Mann et al.
853
Table 2. Radiolucent Lines Zone
Thickness (mm)
No. Patients
1
1 2 1 2 1 2 1 or 2
11 3 2 0 8 2 2
2 3 All 3 zones
Revisions A total of 8 revisions have been done from the original 173 prostheses (4.7%). The underlying causes for the revisions were as follows: infection, 1 (0.6%); augmentation for dislocation, 2 (1.2%); polyethylene wear without loosening, 3 (1.7%); and aseptic loosening, 2 (1.2%). At the time of this writing (2001), another patient has undergone a revision for excessive polyethylene wear, and this revision has undergone a later manipulation under anesthetic for dislocation. This patient has been included in the aforementioned figures. Two patients underwent revision for polyethylene wear, both of whom had stable metal backs. One patient underwent simple liner exchange, and the other patient underwent revision of the metal back because of damage to the metal back by the prosthetic femoral head—mode 4 wear [16]. Despite this wear, the HA-coated metal back remained firmly fixed and was not easy to remove.
There are a further 5 cups that meet our criteria for radiographic loosening—a radiolucent line of ⱖ2 mm over all 3 zones of Charnley and DeLee. If these cups are included, the most pessimistic revision rate for aseptic loosening is 4.1%.
Fig. 1. A 2-mm radiolucent line around entire prosthesis.
Fig. 2. Osteolysis in zone 2.
Polyethylene Wear Review of the radiographs revealed that 7 cups from the 142 radiographs available showed signs of significant radiographic wear (5%) on visual examination. These patients are asymptomatic, however, and there are no signs of actual or impending loosening. Migration Four cups had migrated. Of these 4 cups, 3 showed signs of polyethylene wear. In all cases, the migration subsequently had ceased and no patient had undergone revision for migration alone.
854 The Journal of Arthroplasty Vol. 17 No. 7 October 2002
Discussion The long-term problems affecting the longevity of THAs today include aseptic loosening and polyethylene wear, and these phenomena generally are believed to be linked [16 –26]. We believe that the use of HA, which is osteoinductive and osteoconductive, provides an optimal environment for bonding between host– bone and prosthesis. This bond is such that even in the presence of significant wear debris, the prosthesis does not always become significantly loose (as shown by the low revision rates for aseptic loosening). This bond offers a significant advantage over cemented cups in which revision of the polyethylene insert for wear necessitates revision of the entire prosthesis. It has been suggested by other authors that polyethylene wear may be more problematic in uncemented than cemented cups because owing to the metal back, contact stresses in the polyethylene are higher [27,28], leading to higher wear rates. This problem may be exacerbated in screw cups because difficulty in medializing and inferioralizing the prosthesis would lead to increased shear strains in the polyethylene, adding to the potential for adhesive wear. Excessive polyethylene wear has been described in uncemented screw cups, but we believe that this may have been due to the use of a large proportion (83 of 94) of stainless steel femoral heads [29]. Polyethylene wear remains an issue in uncemented joint arthroplasty, and thin polyethylene liners generally should be avoided. HA-coated acetabular prostheses or femoral prostheses may add to the overall wear debris by contributing thirdbody wear particles. Distinguishing the relative contributions of the third-body component from the inevitable wear from any bearing surface is problematic. Although 4 screw cups had not been medialized adequately, which led to leg-length discrepancy, this was a clinical problem in only 1 of these patients. In all cases, the discrepancy was ⬍1 inch. Radiolucent lines were 7 times more frequent in zone 1 than zone 2. They were 5 times more frequent in zone 3 than zone 2, which is in agreement with other authors [30]. Osteolysis was not common so that any inference in regard to zonal distribution is speculative, although it was most frequent in zone 2.
Conclusion From this midterm follow-up study, we conclude that HA may be beneficial. If screw cups are to be
Fig. 3. Poor anterosuperior coverage.
used, HA coating is mandatory. It would seem that the low level of polyethylene wear seen in this series is likely to be due to the use of a ceramic head. The ceramic/polyethylene couple is known to produce a low penetration rate compared with steel/polyethylene or cobalt-chrome/polyethylene couples [31]. For this reason, we advocate the use of a ceramic head, particularly in young people and especially when uncemented acetabular components are considered. At our institution, however, we no longer use a screw cup despite the low revision rates, owing to perioperative difficulties in achieving the correct offset and obtaining full anterosuperior prosthetic coverage (Fig. 3). If these technical difficulties were addressed, we would be more confident in recommending a HA-coated screw cup prosthesis.
References 1. Capello WN, Colyer RA, Kernek CB, et al: Failure of the Mecron screw-in ring. J Bone Joint Surg Br 75:835, 1993 2. Savilahti S, Myllyneva I, Lindholm TS, et al: Clinical outcome and survival of Link RS total hip prosthesis. J Bone Joint Surg Br 77:369, 1995 3. Simank H-G, Brocai DRC, Reiser D, et al: Middleterm results of threaded acetabular cups. J Bone Joint Surg Br 79:366, 1997 4. Rokkum M, Brandt M, Bye K, et al: Polyethylene wear, osteolysis and acetabular loosening with an HA-coated prosthesis. J Bone Joint Surg Br 81:582, 1999 5. Barrack RL, Mulroy RD, Harris WH: Improved cementing techniques and femoral component loosening in young patients with hip arthroplasty. J Bone Joint Surg Br 74:385, 1992
Hydroxyapatite-Coated Screw Cups 6. McNally SA, Shepperd JAN, Mann CV, Walczak JP: The results at nine to twelve years of the use of a hydroxyapatite-coated femoral stem. J Bone Joint Surg Br 82:378, 2000 7. Callaghan JJ, Forest EE, Olejniczak JP, et al: Charnley total hip replacement in patients less than 50 years old. J Bone Joint Surg Br 80:704, 1998 8. Dorr LD: Fixation of the acetabular component: the case for cementless bone ingrowth modular sockets. J Arthroplasty 11:3, 1996 9. D’Antonio JA, Capello WN, Jaffe WL: Hydroxlyapatite-coated hip implants: mulitcentre three-year clinical and roetgenographic results. Clin Orthop 285: 102, 1992 10. Capello WN, Colyer RA, Kernek CB, et al: Failure of the Mecron screw-in ring. J Bone Joint Surg Br 75:835, 1993 11. Harris WH: The first 32 years of total hip arthroplasty: one surgeon’s perspective. Clin Orthop 274:6, 1992 12. Geesink RGT, Hoefnagels NHM: Six year results of hydroxyapatite-coated total hip replacement. J Bone Joint Surg Br 77:534, 1995 13. Merle d’Aubigne´ R, Postel M: Functional results of hip arthroplasty with acrylic prosthesis. J Bone Joint Surg Am 36:451, 1954 14. DeLee JG, Charnley J: Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 121:20, 1976 15. Freeman MAR: Radiolucent lines: a question of nomenclature (editorial). J Arthroplasty 14:1, 1999 16. Schmalzried TP, Callaghan JJ: Wear in total hip and knee replacements. J Bone Joint Surg Am 81:115, 1999 17. Morscher EW: Current status of acetabular fixation in primary total hip arthroplasty. Clin Orthop 274: 172, 1992 18. Devane PA, Robinson EJ, Bourne RB, Rorabeck CH: Measurement of polyethylene wear in acetabular components inserted with and without cement. J Bone Joint Surg Am 79:682, 1997 19. Shih C-H, Lee P-C, Chen J-H, et al: Measurement of polyethylene wear in cementless total hip arthroplasty. J Bone Joint Surg Br 79:361, 1997 20. Schmalzried TP, Kwong LM, Jasty M, et al: The
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