Short versus conventional stem in cementless total hip arthroplasty

Short-stemmed implants have accompanied the worldwide trend to minimally invasive surgery in total hip arthroplasty (THA) [2, 28] and is increasingly the procedure of choice for hip surgeons in Germany today. Data reported by the German Arthroplasty Registry (EPRD) reveal a relatively high proportion with more than 10% of short stem usage [7]. The first of these to arrive on the market was the Mayo in 1984 in the USA [2]. Over the years there have been many design modifications, and some of the original implants have been superseded by newer designs. There have been several attempts to develop a concordant classification system for short stem femoral components [8, 9, 18, 35], many of which have been modified as the designs themselves have been adapted. Overall, the literature lacks consistency in defining short stem femoral components, complicating scientific follow-up. For the purposes of this analysis, we have used the EPRDs product library definition of short stems as being short cementless femoral stems designed for metaphyseal fixation currently in use and being followed up in the EPRD.

Experience in the EPRD has been encouraging with lower crude overall revision rates when compared with conventional stems [7]. Controversially, there are scientific and anecdotal reports showing increased incidence of aseptic loosening, implant migration and periprosthetic fractures associated with the use of short-stemmed femoral components [1, 2, 11, 19, 20, 31, 32]. Furthermore, the neck-sparing resection and overall bone conserving design was perceived by surgeons as being particularly advantageous in younger patients, resulting in a patient selection bias in many of the larger studies published to date [22, 24]. Currently there are scant large volume population-based studies or national registry data available validating the use of short stems [1, 4, 10, 13]. To evaluate the comparative survivorship between cementless conventional stems and short stems we followed two matched cohorts from the EPRD for up to 5 years for the primary endpoint of revision for any reason. Patients were propensity score matched (PSM) for variables including age and gender. Furthermore, the incidence of prosthetic joint infection (PJI) and of aseptic revision (reasons other than PJI) was evaluated. To account for specific implant design a crude subanalysis of survival of the four most commonly implanted short stems in EPRD was performed.

The crude data demonstrate a selection preference for short stems for younger patients aged 63 years (interquartile range, IQR, 56–69) versus 69 years (IQR 62–76) for the conventional stem group. Following PSM 1:1 on patient, clinic and implant variables, we arrived at two comparable cohorts based on treatment allocation (Table 1).

The crude CPR demonstrates a survival advantage for short-stemmed THAs (P-value log-rank test less than 0.0001). For instance, 5 years after primary THA short stems present a CPR of 2.9%, 95% confidence interval (CI) 2.4–3.5% versus conventional stems with 3.4%, 95% CI 3.2–3.5%; however, after PSM at 1:1 there was no significant difference between the balanced cohorts. In total 353 short stems and 411 conventional stems had been revised. The performance of the matched group of conventional stems improved to approach that of the short stems. At 5 years the conventional stem group demonstrated CPR with 3.1% (95% CI 2.7– 3.4%) (Fig. 2a,b).

The matched CPR for revision reasons other than PJI showed no significant difference between short and conventional stem groups. At 5 years short stems demonstrated an incidence of 2.2% (95% CI 1.7–2.7%) when compared with conventional stems with 2.1% (95% CI 1.8–2.4%). In contrast, the matched CPR due to PJI showed a significant difference resulting in lower incidence in the short stem group. At 5 years short stems demonstrated an incidence for PJI being 0.7% (95% CI 0.5–1.0%) when compared with conventional stems with 1.0% (95% CI 0.8–1.2%) at 5 years after primary THA (Fig. 3).

A crude subanalysis based on the 4 most commonly implanted short stems was performed presenting a CPR range from 1.8% (95% CI 1.5–2.2%) for the Optimys to 3.7% (95% CI 2.9–4.6%) for the Metha stem (Table 2).

Considering the influence of clinic experience with short stems (clinic expertise) we looked at the proportion of short to conventional stems in the hospitals. It was noted that there was variation amongst the most frequently implanted stems with respect to the volume: in clinics where the A2-Kurzschaft and the Optimys stem were used short stems were implanted in 42% and 40% of all THAs, respectively, whereas in clinics where Nanos and Metha stems were implanted, there was less volume for short stems (26% and 15%, respectively).

The most important finding of this study was that there is no survival difference in uncemented THA between comparable groups of short and conventional stems.

Short stem THA has become increasingly popular in the recent years but to date remains unsupported by large population or registry studies. There remains some controversy in the selection of this stem type in high-risk categories, such as older osteoporotic women [20, 27]. Short stems have gained acceptance for use in younger patients [22, 24] as it is perceived that younger patients can benefit from bone and tissue conservation in case of the need for future revision surgery [2, 6, 34, 37]. Due to the resulting patient selection bias between short and conventional stemmed THAs, we aimed to harmonize patient-related confounding factors by excluding patients with known relevant prior operations, complex acetabular bone deficiencies, osteoporosis etc. (Fig. 1) as well as using PSM to balance comparison groups for the remaining confounders age and sex (Table 1).

One of the major factors complicating the evaluation of the literature regarding short stems is the changing definition of what constitutes a short stem and which implants fulfil this definition. The classification of Feyen and Shimmin focused on the length of the stem and fixation in bone [9]. Jerosch described the resection height of the femur and differentiated neck retaining, partial retention and neck resection designs [18]. Van Oldenrijk et al. used a three-stage classification of the collum, partial collum and trochanter-sparing stem designs in their review [35]. With the trochanter-harming short stem Falez et al. additionally introduced a fourth category of short stem femoral components [8].

We based our practical definition of short stem on the current designs in use documented in the EPRD with the design specification of a metaphyseal anchoring cementless femoral component with a CT length of 120 mm or less for at least the smallest implant size. This definition is comparable and overlapping with that of the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) who documented in their 2020 annual report short stems in current use.

Despite the difficulty in defining short stems, there are several studies comparing the clinical and radiological outcome, primary stability and bone remodelling of short stem versus conventional stem THA showing comparable results with follow-up times up to 10 years [12, 21, 36]; however, most of these studies have a small sample sizes, are confined to the analysis of only one stem design and are not compared against the respective national revision rate. A few literature reviews and meta-analyses have focused on bigger population groups: Huo et al. in their meta-analysis reviewed six randomized controlled studies involving 572 THAs demonstrating comparable clinical and radiological outcomes for short and conventional femoral implants [13]. Banerjee et al. in their systematic review of 22 papers addressing the clinical and radiological outcomes for the 9 FDA approved short stem femoral components available in the USA showed good clinical outcomes and implant survivorship in over 99% of cases for short stem THA with mean follow-up for up to 9.8 years in 2734 hip arthroplasties [2] but only 1 of the stems studied is currently in follow-up in the short stem group under the EPRD. Van Oldenrijk et al. in 2014 analyzed 49 international studies including 6495 patients with 19 different short-stemmed implants. Survival rates from 90–100% for short stem THA could be demonstrated with follow-up ranging from 3 to 134.4 months. There was one exceptional study included in this systematic review with 62% survival at 72 months from Ishaque et al. 2009 but this study compared the Thrust Plate Prosthesis with the Eska cut prosthesis, neither of which remain on the market today [15, 35]. The authors concede that their results are not comparable to national registry data because most of the reports are submitted from hip arthroplasty specialist centers, resulting in an expertise bias.

There is evidence that the experience of a hospital with arthroplasty procedures (case load) has a significant influence on the short-term outcome [7, 33]. To take this into account for our study we only included data from hospitals with information about their annual case load of primary THAs, clustered them into three groups of low, medium, and high-volume clinics and arrived after PSM at comparable proportions for the matched groups. The outcome of specific brands (Table 2) seems to be related to the frequency of short stem use within each clinic demonstrating the value of routine use. Routine use must be taken into consideration when evaluating specific designs, as this can result in an expertise bias of the data.

Our results published here are otherwise comparable to other registry literature on this topic. The publication from the Register of Orthopaedic Prosthetic Implants (RIPO, Emilia-Romagna, Italy) reporting on short-term mid-term and long-term (0–17 years) demonstrated a survival rate of 97.4–98.0% at 3 years, with no significant difference to the comparison standard long stem group [10]. Also discussed in the RIPO study is that some short-stemmed implants may demonstrate a higher than expected dislocation rate, either as a consequence of malpositioning, fracture, or subsidence This may be related to the implants themselves, or to the associated surgical (minimally invasive) approach.

A comparison of registry data between the Australian and German registries is complex. A year on year increase in uptake of short stems has been reported by the AOANJRR. In their most recent report 1222 short stems were implanted representing a yearly increase of short stem usage of 8%; however, this still represents less than 1.5% of all primary THAs excluding resurfacing [1]. In comparison, short stems constitute more than 10% of all primary THAs implanted in 2019 as published in the 2020 EPRD annual report [7]. The CPR reported in the AOANJRR 1 year after surgery was 1.8% for short stems compared to 1.6% for conventional stems. At 2 years, the CPR of short stems slows and remains comparable to conventional stems so that at 10 years, the CPR with hazard ratio adjusted for age and sex is 4.9% (95% CI, 3.6–6.6%) for short stems compared to 5.0% (95% CI, 4.9–5.1%) for conventional stems. The shape of the CPR curve is comparable to the results of this study, although it should be noted that the AOANJRR compares uncemented short stems with all conventional stems (cemented and uncemented). Despite the differences in uptake of short-stemmed implants between the two registries, the short-term revision rates are comparable. Ultimately long-term observations with higher numbers of short stem THA will be necessary to elucidate the difference between short and uncemented conventional stem THA.

Thus, the EPRD analysis of the survivorship up to 5 years of short-stemmed femoral components in a relatively high volume over a broad age distribution shows that the performance achieved by these implants, was comparable to conventional cementless femoral components regardless of their individual design attributes. Analyzing 131,580 THA procedures followed up in the EPRD, there was no discernible disadvantage with respect to cumulative revision rates of the short stem THA compared to conventional stem.

To approximate implant-related reasons for revision we analyzed the incidence of aseptic revision between groups in comparison with the incidence of PJI. Early revision for PJI is seen less amongst those receiving short uncemented stems (Fig. 3). There are several potential explanations for this, including remaining patient selection bias (e.g. lower BMI, general health status) and expertise bias. Clearer elucidation of these points will be the focus of future studies of the EPRD. The analysis presented in this study is inadequate to provide a precise and concluding statement regarding the relative risk of PJI due to the prevalence of confounding factors. Although on the whole, we anticipate that with up to 5 years of follow-up, we have captured the most frequent mechanical or design-related reasons for short-stemmed implant failure.

In this large cohort study, we were able to minimize bias by balancing confounding factors within comparison groups, showing similar survival for short and conventional stem THA. There was a significantly lower incidence of PJI for short stems, although we still cannot exclude a certain bias such as remaining patient selection or expertise bias.

We expect that the future behavior of short-stemmed THAs in this cohort of patients will further mimic that of conventional uncemented stems; however further long-term studies are required to rule out late mechanical failure of short stems.