Fixation and loosening of the cemented Müller straight stem

Transcription

1 Fixation and loosening of the cemented Müller straight stem A LONG-TERM CLINICAL AND RADIOLOGICAL REVIEW M. Clauss, M. Luem, P. E. Ochsner, T. Ilchmann From Kantonsspital Liestal, Liestal, Switzerland The original forged Müller straight stem (CoNiCr) has shown excellent ten- to 15-year results. We undertook a long-term survival analysis with special emphasis on radiological changes within a 20-year period of follow-up. In all, 165 primary total hip replacements, undertaken between July 1984 and June 1987 were followed prospectively. Clinical follow-up included a standardised clinical examination, and radiological assessment was based on a standardised anteroposterior radiograph of the pelvis, which was studied for the presence of osteolysis, debonding and cortical atrophy. Survival of the stem with revision for any reason was 81% (95% confidence interval (CI), 76 to 86) at 20 years and for aseptic loosening 87% (95% CI, 82 to 90). At the 20-year follow-up, 15 of the surviving 36 stems showed no radiological changes. Debonding (p = 0.005), osteolysis (p = 0.003) and linear polyethylene wear (p = 0.016) were associated with aseptic loosening, whereas cortical atrophy was not associated with failure (p = 0.008). The 20-year results of the Müller straight stem are comparable to those of other successful cemented systems with similar follow-up. Radiological changes are frequently observed, but with a low incidence of progression, and rarely result in revision. Cortical atrophy appears to be an effect of ageing and not a sign of loosening of the femoral component. M. Clauss, MD, Orthopaedic Surgeon M. Luem, BSc, Clinical Research Associate P. E. Ochsner, MD, Professor, Former Head of Department T. Ilchmann, MD, PhD, Orthopaedic Surgeon Department of Orthopaedic Surgery Kantonsspital Liestal, Rheinstrasse 26, CH 4410 Liestal, Switzerland. Correspondence should be sent to Dr M. Clauss; martin.clauss@ksli.ch 2009 British Editorial Society of Bone and Joint Surgery doi: / x.91b $2.00 J Bone Joint Surg [Br] 2009;91-B: Received 6 November 2008; Accepted after revision 7 April 2009 The original forged Müller straight stem (Zimmer, Winterthur, Switzerland) (cobalt and nickel chromium), introduced in 1977, has shown excellent results at ten to 15 years. 1-4 More than one million implants have been inserted to date, but long-term (> 15 years) survival data are only available from Scandinavian hip registries, 2-4 which lack detail on the clinical or radiological outcome. The Müller straight stem should be compared with the Charnley hip replacement (DePuy, Leeds, United Kingdom), which is widely regarded as the benchmark prosthesis, with a survival of 96% at 25 years, 5 93% at 30 years and 88% at 32.3 years. 7 The long-term success of a cemented stem depends on the longevity of the cement-bone and the cement-prosthesis interfaces. Two different concepts, load-tapered (or forceclosed ) and composite-beam (or shapeclosed ), are described for securing a cemented stem in the proximal femur. 8 The Müller straight stem was designed to achieve a pressfit fixation in the anteroposterior (AP) radiological view with a self-centering effect (shapeclosed). A close stem-bone contact is established in the coronal plane, resulting in a thin or even incomplete cement mantle, 1,9,10 which has been described in the literature as the French paradox. 10 The limited survival data published for the Müller straight stem shows great variation in outcome: at six to eight years 20.1% of the stems were judged to be at risk for later aseptic loosening 12 in a series for which further follow-up has not been presented. For stems implanted with first-generation cementing technique 13 a ten-year revision rate of 8% and a revision rate of 19.7% at 17 years was observed, whereas when using a secondgeneration cementing technique 13 the revision rate was reduced to 4% at ten years. 2 We have analysed the survival, radiological changes and mode of failure of the Müller straight stem over the long term ( 20 years). Patients and Methods Between July 1984 and June 1987 a total of 191 primary total hip replacements (THRs) were performed at our hospital, of which 165 (161 patients) received the Müller straight stem (CoNiCr) and have been followed prospectively. 3 Compared with our earlier publications 3,14 four additional cases were 1158 THE JOURNAL OF BONE AND JOINT SURGERY

2 FIXATION AND LOOSENING OF THE CEMENTED MÜLLER STRAIGHT STEM 1159 Table I. Diagnosis at operation (n = 165) Diagnosis Number (%) Primary osteoarthrosis 119 (72) Slipped capital femoral epiphysis 10 (6) Femoral neck fracture 8 (5) Avascular necrosis 6 (4) Rheumatoid arthritis 6 (4) Others 16 (9) included, initially being misattributed. All stems had a fineblasted surface with a mean roughness Ra 1.0 μm (0.5 μm to 1.5 μm) and a longitudinal groove both anteriorly and posteriorly. Inspection of implants retrieved during revision revealed two variants of the stem concerning additional fine anterior and posterior longitudinal ribs not realised in former publications. The first 90 stems did not have these ribs, whereas they were present on those used later and are still present in current designs. For eight stems the surface structure could not be determined. The mean age of the patients at operation was 68.9 years (25.6 to 86.3), with 70 stems implanted in women and 96 on the right side. Diagnoses are presented in Table I. All operations were undertaken or assisted by one of four consultants (PEO). The patients were operated on in a standardised way, in a supine position through a lateral transgluteal approach using the largest implant that could be contained in the medullary canal. No trochanteric osteotomy was performed. In contrast to other canal-filling implants, no spongious bone was removed in the sagittal plane. 15 All stems were cemented with a second-generation cementing technique comprising a distal plug, cement syringe, no vacuum mixing, no jet lavage and no proximal sealing using Sulfix-6 bone cement (Zimmer). In all, 96 stems were combined with a cemented polyethylene acetabular component, 53 with a cemented polyethylene acetabular component with additional screws, 14 with an acetabular reinforcement ring combined with a polyethylene component (Müller Ring, Zimmer) and one with an anti-protrusion cage (Burch-Schneider cage, Zimmer) combined with a polyethylene component. One stem was combined with an uncemented polyethylene acetabular component (Mathys RM cup, Mathys AG, Bettlach, Switzerland). All femoral heads had a 32 mm diameter of which 134 were cobalt-chromium and 31 were ceramic. Clinical and radiological follow-up was at four months and 1, 2, 5, 10, 15 and 20 years. Clinical follow-up included a standardised examination using the Harris hip score (HHS). 16 Radiological assessment was based on a standardised AP radiograph of the pelvis centred on the symphysis, showing the entire prosthesis, and a second plain radiograph with the false profile view. The indication for revision was symptomatic radiological loosening. The films were rated according to the Gruen zone system. 17 The coverage of the tip of the stem with cement was assessed. Varus/valgus alignment of the stem was measured on the four-month post-operative AP radiograph, 18 with a deviation of > 3 defined as malalignment. Subsidence was assessed by measuring the vertical increase of any radiolucency created by distal migration of the shoulder of the prosthesis from any overlapping proximal cement in Gruen zone 1. 1 Debonding was defined as a radiolucent line at the prosthesis-cement interface not visible on the first postoperative radiograph. Osteolysis was defined as progressive, newly developed endosteal bone loss with a diameter > 3 mm, either with scalloping or a bead-shaped lucency at the cement-bone interface. 19 Polyethylene wear was assessed by measuring the minimum thickness on the final radiograph compared with the post-operative films, 20 to permit the assumption of the annual wear rate. The femur was assessed for cortical atrophy, 21 defined as a longitudinal intracortical porosis with a consecutive thinning of the cortex without measurable thickening of the femur. The date of the first radiological appearance was noted for all radiological changes. Statistical analysis. All data are presented as a mean with an SD. The Kolmogorov-Smirnov test was used to test whether the data were normally distributed. As most data were normally distributed, we used the t-test to compare data analysis in two unpaired groups. In the groups with binomial results we used Fisher s exact test. Correlations between single parameters were examined with Pearson s correlation coefficient and survival analysis was conducted using the Kaplan-Meier method, including the 95% confidence intervals (CI). A p-value < 0.05 was considered statistically significant. Statistical analysis was performed with use of the SPSS statistical package v 13.0 (SPSS Inc., Chicago, Illinois). Results Follow-up. From the 161 patients (165 stems) two patients were lost to follow-up, with their last review at 4.2 years and 10.5 years after operation, respectively, both without revision. In all, 46 patients (46 hips) died before the tenyear review at a mean of 5.6 years (0 to 9.9). The original radiographs of these patients were destroyed, but clinical data and copies of the destroyed radiographs which had been microfilmed were available, and none of them was scheduled for revision. Owing to the short follow-up these patients were not used for the long-term radiological study. For seven stems the radiograph of the final follow-up was lost or of insufficient quality, leaving 110 complete radiographic series of femoral components for detailed longterm analysis. The analyses were done for the whole cohort and separated for the two different macroscopic finishes. However, as we found no influence of the topographical features on any of the clinical and radiological results, we present only the results for the whole series. Out of all 161 patients (165 stems), 106 (110 stems) died within the 20 years follow-up. Of these, 17 hips of the VOL. 91-B, No. 9, SEPTEMBER 2009

3 1160 M. CLAUSS, M. LUEM, P. E. OCHSNER, T. ILCHMANN Fig. 1 Kaplan-Meier survival curve of the Müller hip replacement. Fig. 2 Serial radiographs of a man aged 56 years at operation who received a Müller straight stem with a cemented polyethylene acetabular component and a cobalt-chrome head. The stem had a neutral axis with the tip not covered by cement. Continuous subsidence is observed from ten years to 18.1 years with development of osteolysis in Gruen zones 1 to 7, debonding in Gruen zones 1 and 2, polyethylene wear of 2.7 mm (wearrate 0.15 mm/year) and evidence of acetabular component loosening. Both components were revised. 165 hips (17 patients) had a complete revision: ten for aseptic loosening after a mean of 13.4 years (7.2 to 18.3), three due to late infection (6.7, 18.1 and 20.3 years, respectively, after operation) and four for other reasons (two for recurrent dislocation, and two for wear. A total of 11 of the 165 hips (11 patients) had an isolated acetabular component revision after a mean of 12.0 years (5.2 to 20.0); no isolated femoral components were revised. In all, 36 hips (33 patients) had a femoral component survival of a minimum of 20 years, nine of these had an isolated acetabular component revision. With femoral component revision for any reason, survival after 20 years was 81% (95% CI 76 to 86) and for aseptic loosening of the femoral component alone the survival was 87% (95% CI 82 to 90) (Fig. 1). Clinical results for the 36 hips after 20 years. The mean HHS was 79 (38 to 100), of which eight (22%) of the hips showed excellent (90 to 100 points), ten (28%) good (80 to 89 points), 12 (33%) fair (70 to 79 points) and six (17%) poor results (< 70 points) at the last follow-up. A total of 25 hips were free of pain; ten showed only slight pain. The mean HHS was 78 (38 to 98) for the 27 hips without any revision and 82 (61 to 92) for the nine hips with an isolated acetabular component revision, with a mean pain score of 42 (35 to 44) in both groups. Radiological results for the 110 hips with at least ten years survival. In eight stems (7%) the tip was not covered with cement: these stems showed no statistically significant difference in survival (p = 0.994). A total of 99 of the stems (90%) had a neutral axis post-operatively, but long-term survival ( 20 years) was not statistically significantly related to the initial stem axis (p = 0.305). Nine stems (8%) subsided > 2 mm; four of these were revised for aseptic loosening. Subsidence of > 2 mm was found to be associated with an increased likelihood of revision (p = 0.004). All subsidence occurred within the cement mantle and for all subsiding stems the cement-bone interface seemed intact apart from occasional areas of osteolysis. Debonding was seen in 17 of the 110 hips after a mean of 14.5 years (0.9 to 22.2) (Fig. 2), being always located on the superolateral aspect of the stem. A total of seven of the 36 hips with a 20-year follow-up and seven of the ten stems revised for aseptic loosening showed debonding. This was statistically significantly different from the stems which had survived (p = 0.005). Osteolysis was found in 40 (36%) of the 110 analysed hips (Fig. 2), mainly on the proximal-medial side of the stem (Gruen zones 7 and 6) and rarely laterally (Gruen zones 1 to 3). It was always visible on the AP radiograph first, and there was no stem with initial osteolysis on the lateral plane. A higher risk of osteolysis was correlated with a younger age at operation (p = 0.001) and male gender (p = 0.046). In the 40 hips with detectable osteolysis it appeared after a mean of 13.1 years (SD 4.0) with 12 of 36 hips having a survival of a minimum of 20 years exhibiting detectable osteolysis after a mean of 15.2 years (SD 4.8). In nine of the ten stems revised due to aseptic loosening osteolysis was present and had appeared earlier at a mean of 12 years (SD 4.6) and was statistically more frequent (p = 0.003) and involved a greater area around the stem ((mean 4.2, SD 2.5) Gruen zones involved versus mean 0.7 (SD 1.3), (p = 0.002)) than in the surviving stems. The mean polyethylene wear rate for all 110 hips was 0.07 mm/year (SD 0.071). The stems with osteolysis had a mean wear rate of 0.10 mm/year (SD 0.077), whereas those without osteolysis had a mean wear rate of mm/year (SD 0.056). The highest wear rates were found for hips requiring femoral revision (mean 0.14 mm/year) (SD 0.087). The 36 hips with > 20 years follow-up had a THE JOURNAL OF BONE AND JOINT SURGERY

4 FIXATION AND LOOSENING OF THE CEMENTED MÜLLER STRAIGHT STEM 1161 mean wear rate of mm/year (SD 0.05). Polyethylene wear correlated with osteolysis (p = 0.001) and revision of the stem (p = 0.016). Wear rates were lower for the 82 acetabular components without revision (mean mm/year, SD 0.062) than for the 28 revised acetabular components (0.12 mm/year, SD 0.082, p = 0.003). Aseptic acetabular component revision was associated with osteolysis around the stem (p = 0.018). Polyethylene wear was higher for the 32 mm CoCr heads (mean mm/ year, SD 0.074) than for the 32 mm ceramic heads (mean mm/year, SD 0.057, p = 0.024) but hips with CoCr heads had no increased association with osteolysis (p = 0.255) or stem revision (p = 0.446). Of the 36 stems with a 20-year survival, 17 showed cortical atrophy, seen most often in Gruen zones 2 and 6, followed by zones 3 and 5 (Fig. 3). Except for one stem, cortical atrophy was first detectable in Gruen zones 2 or 6, and there was no stem with cortical atrophy before the tenyear follow-up. It was found that 13 of 16 female and four of 20 male patients had cortical atrophy after at least 20 years of follow-up (p = 0.001), but cortical atrophy did not correlate with age at operation (p = 0.222) but with age at final follow-up (p = 0.01) and duration of follow-up (p = 0.008). The ten stems revised for aseptic loosening did not show cortical atrophy, and this feature was not itself associated with aseptic loosening (p = 0.008). A total of 40 (36%) of all 110 stems had no radiological changes, and 23 (21%) had only cortical atrophy. The remaining 47 (43%) showed debonding and/or osteolysis. Of the 36 stems with at least 20 years of follow-up, nine (25%) had no changes, 17 (47%) only cortical atrophy and ten (28%) debonding and/or osteolysis. Discussion With aseptic loosening as the endpoint, the survival of the Müller straight stem at 20 years was 87%; thus we were able to show its reliability in the long term. 1,2,4,9,10,12,22 This survival rate is comparable to that of other well known and successful cemented systems in larger multi-surgeon series, 2,4 but does not match the results of the best published single surgeon series from specialist centres. 5,6 Clinical results. Our clinical results are comparable to the long-term results of other series. The reduction in functional status of patients receiving THR is a normal development with time, being mostly influenced by comorbidities in the case of stable implants. 23 Isolated acetabular component revision did not affect clinical long-term results, with 70% of surviving patients still free of pain in the operated hip after 20 years. Radiological results. Krismer et al 12 found an increased rate for aseptic loosening for Müller straight stems with an incomplete cement mantle at the tip of the stem. In our series, cement defects at the tip of the stem were uncommon and not associated with an increased incidence of revision. The stem design and implantation technique make gross errors in varus-valgus orientation almost impossible, as the Fig. 3 Serial radiographs of a man aged 76 years at operation with the same choice of component as in Figure 2. At ten years cortical atrophy was first noted in Gruen zones 2 and 6. At 20.5 years the patient remained pain free. Cortical atrophy was present in all Gruen zones without any debonding, subsidence, or osteolysis. The polyethylene wear was 0.8 mm (wear rate 0.04 mm/year). The numbers shown in the right image indicate the frequency of cortical atrophy in the various Gruen zones for stems with 20 years follow-up (n = 36). prosthesis almost fills the femoral canal in the AP plane. Thus we found only a few stems with varus-valgus malalignment. In the lateral plane, cemented straight stems show a typical pattern of cement distribution, with implant-bone contact in the anteroproximal and posterior-distal area. 24 Initial osteolysis, when it occurred, appeared first on the AP radiographs where the cement mantle was thin or even incomplete, and osteolysis in the sagittal plane appeared late if at all. Thus we conclude that lateral views of the stem might be important for following cemented stems fixed in the force-closed manner, 8 but seem to be unnecessary for cemented stems fixed in a shape-closed manner such as the Müller straight stem. This stem had a satin surface finish (Ra 1.0 μm), exceeding a postulated roughness of 0.4 μm defined as maximum roughness for polished stems. Thus abrasive wear of the surface and a high volume of metal debris might be expected. 24 Later versions of the stem had an even higher surface roughness and survival decreased. 14 The combination of the soft metal titanium with a rough surface had the worst results. 26 The detailed experience of our institution with various versions of this implant has recently been published. 27 It is well established that polished stems have a better survival with force-closed cementing technique. 28 Shape-closed implants worked best with a polished surface (French paradox), 11 the published results were superior to our presented series with a satin surface finish, 25 and polishing of the surface seems to improve the long-term survival of any kind of cemented implant. 29 The Swedish Hip Registry 2 showed improved long-term survival for the Müller straight stem using a secondgeneration cementing technique compared with a series with first-generation cementing technique without a distal VOL. 91-B, No. 9, SEPTEMBER 2009

5 1162 M. CLAUSS, M. LUEM, P. E. OCHSNER, T. ILCHMANN plug. There are no data for third-generation techniques (jet lavage, vacuum-mixed cement), and it must be questioned whether these techniques can further improve the cement penetration for an implant with high introduction forces. Subsidence was not a particular problem in our and other series, 22 using a second-generation cementing technique, and only occurred within the cement mantle. It was frequently observed in cases of first-generation cementing, with stems migrating in conjunction with the cement mantle, indicating a weak bone-cement interface. 1 Debonding of the straight stem cemented with the second-generation technique is a known phenomenon 12 and a risk factor for aseptic loosening. In first-generation cementing technique it is not observed, 1 probably due to subsidence of the cement itself. The presence of wear was associated with osteolysis and revision, with the lowest rate of wear being found for the implants without any radiological changes, comparable to other series of patients with long-term well-functioning implants without radiological changes. 30 Polyethylene wear is a well-known risk factor for osteolysis and aseptic loosening, especially with an incomplete cement mantle. 31,32 The use of ceramic heads was associated with a decreased wear rate and the use of modern bearing surfaces might further improve survival. 1,9,10,12 The use of a femoral seal or finger-packing might improve the proximal sealing and reduce access of polyethylene particles at the interface, thus reducing the risk of osteolysis for the Müller straight stem. However, the biological effect of the abrasive wear of the rough stem might be equally important. 29 This cannot be overcome by modifications to the cementing technique, 24 and can only be overcome by polishing the stem. Smith and Walker 21 described an age-related expansion of the human proximal femur in a series of 2300 healthy female femora and postulated that endosteal resorption would result in an expansion of the medullary canal, which might even occur after insertion of a THR. 33 A time- and gender-related widening of the medullary canal with consecutive thinning of the cortex has also been reported in female cadaver femora of various ages. 34 Radiologically, an obvious loss of mineralisation of the cortex and cancellous bone has been observed in older women. 35 Our cortical atrophy, found mostly in the group of elderly women and usually starting in Gruen zones 2 and 6, corresponds to these observations. Räber et al 1 reported a 15-year survival rate for aseptic loosening of 88.1% with the Müller straight stem when using a first generation cementing technique, but found that about 70% of the remaining stems exhibited osteolysis or longitudinal lucencies. The incidence of osteolysis was not reported, but the longitudinal lucencies might have been due to cortical atrophy. In our series the incidence of any radiological changes was common, but only osteolysis and debonding were associated with a risk of loosening. However, awareness of the natural process of cortical atrophy is necessary in order not to overestimate the number of cases at risk, as cortical atrophy did not compromise the clinical and radiological results. After a mean of 20.5 years we found a survival rate for aseptic loosening of 86% (95% CI 82 to 90) with the cemented Müller straight stem, comparable to other wellfunctioning cemented systems. Osteolysis and/or debonding were found in 43%, which correlated with aseptic loosening. The results might be amenable to further improvement by the use of more wear-resistant bearing surfaces, a distal plug in combination with improved proximal sealing and a polished surface finish of the stem. In all, 70 (64%) of the 110 stems showed radiological changes within 20 years, but cortical atrophy was a frequent radiological phenomenon in the long term, being an effect of ageing and not related to aseptic loosening. We thank Mrs S. Häfliger for her exceptional effort in organising the follow-up examinations of our patients for the last 25 years. Although none of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received but will be directed solely to a research fund, foundation, educational institution, or other non-profit organisation with which one or more of the authors are associated. References 1. Räber DA, Czaja S, Morscher EW. Fifteen-year results of the Muller CoCrNiMo straight stem. Arch Orthop Trauma Surg 2001;121: Malchau H, Herberts P, Södermann P, Oden A. 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