There have been many modifications to the design of total hip prostheses and to the concepts regarding fixation of the components. This is especially true in regard to the acetabular cup, with many surgeons now using a cementless acetabular component as part of a totally uncemented hip replacement or as part of a so-called hybrid hip.1
Despite improvements in design and fixation methods, one of the most common problems still encountered in total hip replacement (THR) is aseptic loosening of the components, particularly of the cup.2 Although the incidence of cup loosening varies considerably, one large representative series of cemented acetabular components revealed that 8.4% of patients required cup revision within 10 years and this figure reached 17.5% at 18 years.3
The reported incidence of prosthetic loosening varies according to definition and diagnostic methods, but probably the most accurate definition of a loose prosthesis is one that is migrating, even if there are no clinical problems and radiographs appear normal.4 Recently, it has been shown that acetabular component migration measured at 2 years serves as an accurate predictor of prosthetic longevity.5 Apart from roentgen stereophotogrammetric analysis (RSA), which is expensive and can only be used prospectively, methods of measuring cup migration have hitherto been inaccurate and time consuming. To draw conclusions from such measurements requires large patient numbers and complex statistical analysis.6
This study measured CLS (Cementless Total Hip Replacement System; Protek AG, Bern, Switzerland) acetabular cup migration retrospectively from plain anteroposterior pelvic radiographs and correlated this information with the radiographic appearances and clinical outcome in a series of patients with uncemented CLS THRs.
Fig 1: Reference points used for measuring cup migration.
MATERIALS AND METHODS
The CLS cup is a slightly flattened hemispherical prosthesis made of titanium alloy with a corundum-blasted surface designed to be inserted with press-fit stabilization. The metal back of the cup has six lobes covered in teeth that, when expanded, have an outside diameter greater than the reamer used to prepare the acetabulum. The liner, made of high-density polyethylene (HDPE), is screwed into tfie expansion cup and articulates with a 28- or 32mm ceramic head.
Over a 4-year period, 96 such cementless CLS acetabular components were implanted in 88 patients as part of an uncemented CLS total hip replacement. The patient population was comprised of 44 men and 44 women.
Preoperative diagnoses included osteoarthritis (86 cases), avascular necrosis secondary to intracapsular fractures of the femoral neck (6 cases), and rheumatoid arthritis (4 cases). The mean age was 59.6 years (range: 39.669.4) and mean weight was 74 kg (range: 39-115). Surgery was carried out through a transgluteal approach7 with the patient supine. Postoperatively, patients were mobilized for 8 weeks with weight bearing on crutches.
Migration data were obtained from standard AP pelvic radiographs taken immediately postoperatively, at 3 montiis, at 1 year, and annually thereafter.
Fig 2: Migration of the CLS cup in the cranial (A) and medial (B) directions.
For each radiograph, 11 reference points were plotted - 3 on the cup and 8 on the pelvic bones (Fig 1). The magnification was calculated from the femoral head. Reference points could not be accurately determined in 11 patients, and these patients were excluded.
All points were digitized and the data entered into a computer. Software designed for this study enabled coordinates to be calculated, and all subsequent radiographs were oriented in the same way. The points on the teardrop were used to calculate horizontal migration of the cup while the points on the teardrop and ischium combined were used to determine vertical migration. Radiographs of a cadaveric pelvis with a CLS acetabular cup were taken in the neutral position, 10° of flexion/extension, and 10° of rotation to determine the sensitivity to different orientations of successive radiographs.
All radiographs were reviewed by the first author (M.S.D.). The presence or absence of bone ingrowth and sclerotic or lucent lines adjacent to the prosthesis was recorded. All patients were mailed a clinical questionnaire and asked to comment on any pain experienced and to detail the site, the time of onset, the intensity (visual analogue scale), exacerbating factors, and the frequency of this pain. Patients also were asked to rate the outcome of their THR as excellent, good, fair, or poor. All data were analyzed by a statistician.
The mean follow-up was 2.23 years (range: 1-4.66 years) for radiographic review and 3 years (range: 1-5 years) for clinical review. Table 1 details the number of complete sets of migration data for each year of follow-up.
Methodology. The method used in the study has an accuracy of 0.3 mm in the x-axis (medial migration) and Ö.6 mm in the y-axis (cranial direction).
Migration Results and Radiographic Assessment. Migration data for the medial and cranial directions are plotted in Figure 2. At 2 years, the mean migration was 1.35 mm in the cranial direction and 2.15 mm in the medial direction.
A generalized increase in bone density around the prosthesis (bone ingrowth) was noted in 55 (57.3%) cases, usually within 3 months of surgery and predominantly in DeLee and Charnley zone 1. This was particularly noticeable in the region of the superior petals of the cup where bridging trabecular bone was observed in what appeared to be hollow spaces on the immediate postoperative film.
Sclerotic lines adjacent to the prosthesis were observed in 21 cases (Table 2). All of the sclerotic lines were nonprogressive and <1 mm thick. No lucent lines adjacent to the cup were noted.
Clinical Results. Of the 94 questionnaires mailed, 89 (94.9%) responses were received. Two patients had died of causes unrelated to their THR. The medical records of the 5 patients who did not respond to the questionnaire indicated that thigh pain was absent in these 5 patients.
Three patients indicated that they had thigh pain and five had groin pain. The mean visual analogue pain score was 1.65 (range: 1-3). The onset of pain varied in this group of patients from the first postoperative day to 2 years (mean: 6 months). In all eight cases, the pain was intermittent, and in none of the cases was the pain worsening.
Fig 3: Immediate postoperative radiograph (A) and radiograph taken 1 year postoperatively (B). Bone ingrowth onto the acetabular cup can be noted in the 1-year postoperative radiograph.
Complete sets of migration data
Distribution of sclerotic lines by DeLee and Charnley zones
Four patients indicated that they used a cane, but only for long walks. Overall, 83.1% rated their THR as excellent, 11.2% as good, 5.7% as fair, and 0% as poor.
Statistical Analysis. A significant correlation was found between shorter migration distances in the medial direction and the presence of bone ingrowth (Ps.OOl at 1 year and /HOIl at 5 years; Mann- Whitney U test). There was no such correlation for cranial migration, and there was no association between migration and the presence or absence of sclerotic lines.
Analysis of the remainder of the data showed no statistical relationship between migration of the cup in either the medial or cranial directions and age, weight, or patients' perceived outcome using Spearman's rank correlation test. There was no significant correlation between migration and sex, pain, site, or intensity of pam (MannWhitney U test).
This study retrospectively measured the migration of the CLS acetabular component from standard AP radiographs and correlated the results with radiographic appearances and clinical outcome. The method we used is fast, easy to use, and inexpensive, and approaches the accuracy of RSA. Although our method cannot measure migration in the AP direction, this is not a problem as cup migration principally occurs in the cranial and medial or lateral directions.8
We believe the design concept of the CLS acetabular component to be sound. This is supported by a number of observations. First, the migration profiles in the cranial and medial directions show that migration of the cup slows with time rather than increases. Second, the radiographic appearances suggest that in the majority of cases, there is significant ingrowth of bone onto the prosthesis. This suggests mat stability is achieved by osseous integration. Finally, the clinical outcome study reveals a low incidence of thigh and groin pain when compared with some other uncemented prostheses.9,10
In those cases in which there was no alteration in the trabecular bone pattern adjacent to the cup, there were no radiographic features suggestive of loosening. However, in this group of patients, the migration of the cup in the medial direction was significantly higher compared with the ingrowth group, but interestingly not in the cranial direction. The explanation for this observation is not clear, and only longer term follow-up will show whether there is earlier failure of the cup in this group.
It is also interesting to note that forces across the acetabulum are greater in the cranial than the medial direction, and yet migration of the CLS cup in our study was greater in the medial than the cranial direction. This finding also is not easily explained, but is likely related to the design of the CLS cup.
Although the site of pain in THR patients is not component-specific, the migration of the cup in those patients complaining of thigh or groin pain was not significantly different from those patients with no pain. In addition, there was no correlation between patients' perceived outcome and acetabular component migration. As a result of this observation, we have been unable to show an association between migration of the CLS cup in our series and the pain experienced by a small percentage of die patients.
Other studies of the CLS prosthesis11·12 have shown it to have good short- and medium-term results, and this is supported by our study. The clinical results are satisfactory and the migration profile suggests that the design is inherently stable, In addition, the radiographic appearances altered little over the study period and those features identified were not associated with prosthetic failure. Longer term follow-up is necessary before we will be able to relate our findings to failure.
1. Maloney WJ, Harris WH. Comparison of a hybrid hip with an uncemented total hip replacement. J Bone Joint Surg Am. 1990: 72:1349-1352
2. Morscher EW. Noncementéd acetabular fixation in primary total hip replacement. In: Morscher EW, ed. Endoprosthetics. Springer; 1995:143-179
3. Garcia-Cimbrelo E, Munuera L. Early and late loosening of the acetabular cup after lowfriction arthroplasty. J Bone Joint Surg Am. 1992; 74: 111 9- 1129.
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7. Nazarian S. Tisserand PH, Brunet CH, Müller ME. Anatomical basis of the transgluteal approach to the hip. In: Surgical-Radiologic Anatmoy. Springer- Verlag; 1987:27-35.
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11. Blasius K. Cotta H. Thomsen M. CLS multicentre study: 8 year results. Z Orthop /fire Grenzgeb. 1993; 131:547-542.
12. Spotomo L. Romagnoli S, Ivaldo N, et al. The CLS system. Theoretical concept and results. Acta Orthop BeIg. 1993; 59(suppl 11:144-148.
Complete sets of migration data
Distribution of sclerotic lines by DeLee and Charnley zones