The aim of this study was to accurately characterise the migration pattern of a clinically proven cementless stem over a two-year period using published RSA techniques [10
One limitation of this study was the number of patients in which RSA examinations were incomplete or in which the radiographs were of insufficient quality. Second, although the intended length of follow-up was short, a longer RSA study would be useful to confirm the ability of early RSA evaluation to predict the long-term behaviour of cementless stems.
Our data demonstrate that distal stem migration was confined to the first six postoperative months. Subsidence subsequently stabilised, with no further measurable subsidence between six months and two years (Fig. ). This pattern of early migration has been reported previously with other cementless stems; however, there is limited information in the literature regarding a definitive cause for this early migration [20
]. Impaction of the stem during the early postoperative phase may account for this finding of migration limited to the first six months. While our study did not evaluate the pattern of subsidence within the first six postoperative months, a previous study of migration in a cementless stem showed the majority of stem subsidence occurs within the first two months with no major stem migration occurring within the first week of implantation [22
Subsidence caused by postoperative impaction may be exaggerated by early, unrestricted weightbearing. A study of the influence of postoperative weightbearing on distal stem migration after THA demonstrated that although the degree of loading did not correlate with stem micromotion at two years, immediate unrestricted postoperative loading did increase early subsidence [23
]. Furthermore, Ström et al. [23
] reported that early migration of the cementless CLS stem (Centerpulse, Bern, Switzerland) was consistently followed by stabilisation of the implant, further supporting the notion that the early subsidence measured for the Corail stem is a result of “impaction” rather than “true” subsidence related to implant instability. In addition to the similarities in the pattern of migration between the uncoated CLS stem and the hydroxyapatite-coated Corail stem, the magnitudes of subsidence were also comparable [23
Kärrholm et al. [10
] suggest acceptable early migration (subsidence) for cementless stems is less than 1–1.5 mm within the first two years when measured by RSA. Although the measurements of stem subsidence in this study fall well below this threshold, the authors report subsidence of 0.5–1.0 mm within the first one to two years is cause for concern as a result of the increased risk of clinical failure seen with early migration [10
]. A striking feature of our study is the variability in the magnitude of subsidence between patients measured at six months. Over one third of the patients whose data were included in the results experienced subsidence of over 0.5 mm within the first six months. Although these patients lie within the limit of this accepted range (0.5–1.0 mm), the proven clinical success of this stem suggests one should be cautious in predicting the long-term performance of a stem purely on the numeric basis of its initial subsidence. A possible explanation for the variability in subsidence seen between patients is a difference in the quality of the initial stem fixation. Patients with a stem that is comparatively less impacted at the time of surgery may be expected to have more initial migration than those patients with a firmly impacted stem. Also, differences in the quality of the bone surrounding the prosthesis may account for some of the variability in subsidence.
Krismer et al. [20
] highlighted the importance of the pattern of early migration during evaluation of the ten-year migration patterns of both cemented and cementless stems using the Einzel-Bild-Roentgen-Analyse (EBRA) method. The findings of this study showed a large proportion of the stems demonstrated early subsidence followed by subsequent stability and good long-term survivorship. The authors concluded initial subsidence does not always lead to early revision [20
]. A similar study by Kroell et al. [24
] also evaluated the migration patterns of cemented and cementless stems. In contrast to the study by Krismer et al. [20
], all the stems evaluated were ultimately revised. The results demonstrated that the majority of cementless stems requiring revision had a pattern of continual subsidence, with a smaller portion undergoing late onset migration. Interestingly, none of the stems which ultimately failed demonstrated a pattern of early subsidence followed by stability. Despite the large variation in early subsidence seen in our study, all of the stems subsequently stabilised. This feature of the Corail stem may be related to its design and hydroxtapatite coating and may help to explain its historically good performance clinically. It also highlights the importance of observing the stem subsidence pattern in conjunction with numeric evaluation.
The positioning of markers by the manufacturer on the proximal shoulder and distal tip of the stem enabled stem rotation and tilt to be measured in this study. As with the measurements of subsidence, maximum rotation in this study occurred within the first six months (Fig. ). Evaluation of the stem rotation showed a mean rotation into retroversion of 1.90° at one year. The amount of rotation measured is consistent with reports in the literature from other cementless stems. In their study of early migration of the CLS stem, Ström et al. [22
] found that stems rotated into retroversion on average 1.79° in patients with unrestricted weight bearing after one year. Almost one third of the patients implanted with the Corail stem experienced more than 2° of stem rotation into retroversion. Although this is a large proportion of patients, only one of these patients experienced continued rotation after six months. Again this highlights the ability of the Corail stem to stabilise despite large variability in early rotational stability. The Corail stem demonstrated only a small amount of posterior tilt and varus rotation, which also occurred within the first six months of the study.
Although there is some information regarding the clinical outcome of cementless stems, there is a paucity of information that accurately describes the migration characteristics of a clinically successful cementless femoral stem. From the results of this study, surgeons may expect to find a variable degree of early migration when using the Corail stem; however, in all of the cases presented here this was followed by subsequent stability. The amount of overall migration for the Corail stem is within the limits suggested as acceptable in the literature within the first two years, with negligible subsidence occurring in the final 18 months of the study. We believe that these findings are valuable in creating a standard set of values for the magnitude and pattern of stem migration that is likely to result in a prosthesis that has consistently good clinical outcomes. These data may serve as a benchmark for others to compare novel cementless stem designs.