Abstract
Background
Tapered, fluted, titanium (TFT) stems have shown good clinical outcomes in revision total hip arthroplasty (rTHA), however concerns exist regarding early subsidence. This study compares subsidence between a modern monoblock 3-degree and a modular 2-degree TFT stem in rTHA.
Methods
A retrospective, international multicentre comparative study was conducted including 64 rTHA in 63 patients. A monoblock TFT stem was used in 37 cases and a modular TFT stem was used in 27 cases. Patient demographics, Paprosky femoral bone loss classification, bicortical contact and stem subsidence were recorded at minimum four week follow up.
Results
There was no statistically significant difference in overall subsidence (p = 0.318) or the rate of subsidence >10 mm between stems. Mean subsidence was 2.13 mm in the monoblock group and 3.15 mm in the modular group. Two stems subsided >10 mm: one in each group. There was no difference in bicortical contact between groups (p = 0.98). No re-revisions were performed.
Conclusions
We found no difference in subsidence between the two stems. Surgeons may consider the use of monoblock stems in rTHA as they have comparably low rates of subsidence and eliminate the small but potentially catastrophic risk of implant fracture at modular junctions associated with modular stems.
Keywords
1. Introduction
In the setting of revision total hip arthroplasty (rTHA), achieving stability of the femoral stem is of paramount importance, particularly in cases of deficient proximal femoral bone stock. In these cases, when the metaphyseal bone is insufficient to provide mechanical support, distal fixation with a diaphyseal-bearing stem may be utilized. Tapered, fluted, titanium (TFT) stems have gained popularity in the setting of rTHA as they provide both axial and rotational stability through their design: a tapered profile produces axial stability and longitudinal splines confer rotational stability, while the use of titanium alloys allow load transfer and minimizes stress shielding. Long term stability is achieved through osseointegration
1
TFT stems are not without problems. The original monoblock Wagner design has been associated with subsidence, with studies reporting the rate of subsidence >10 mm between 15 and 20%.
2
, 3
, 4
, 5
Most of this subsidence occurs within the first three months.2
,5
, 6
, 7
, 8
In many cases this may be attributed to surgical technique; component under sizing and a tendency to seat the stem at a level in the femur based on leg length rather than axial implant stability are both potential contributors.9
,10
Furthermore, rates of dislocation as high as 14% have been reported, which may be attributable to loss of hip joint tension following stem subsidence.11
,12
In an effort to combat subsidence as well as improve stability, novel designs incorporated an increased diaphyseal taper angle as well as modular interfaces to allow restoration of leg length and femoral offset. This modularity allows surgeons to seat the stem at a depth based entirely on the stability of the implant within the femur; restoration of leg length, version and femoral offset is possible using modular components following placement of the distal stem. However increased modularity is not without consequence, with reports highlighting the potential for corrosion and fracture causing catastrophic failure of TFT stems.13
, 14
, 15
Fears surrounding failure at modular junctions has led some institutions to favor monoblock designs.1
Furthermore, in many modular systems the offset of the stems is proportional to the size of the proximal body, making restoration of offset difficult in cases with small proximal femora.Thus the ideal femoral component for rTHA in the setting of bone deficiency remains uncertain. Advances in monoblock stem design have shown the potential to improve on previously reported subsidence and instability rates, while avoiding issues at modular junctions.
16
Further advantages may include reduced surgical instrument sets, reduced hospital inventory and subsequent cost reduction. In this study we aimed to compare rates of subsidence and bicortical contact between modular and monoblock TFT stems in rTHA. We hypothesize that the use of a modern monoblock stem will result in comparable rates of subsidence to modular stems.2. Methods
2.1 Patients
This retrospective, international multicentre comparative study was granted institutional review board approval prior to commencement. Inclusion criteria were patients undergoing rTHA in our institution with a monoblock or modular TFT stem between January 01, 2017 and December 31, 2019 with minimum four weeks radiographic follow up (mean follow up: 193 days [range: 28-666]). Patient with less than four weeks radiographic follow up were excluded. Baseline characteristics of all patients were recorded and included age, sex, indication for revision, Paprosky femoral bone loss classification, surgical technique (including use of extended trochanteric osteotomy [ETO]) and implants used. Patients were enrolled in a national or institutional registry depending on center.
2.2 Monoblock stem group
The Redapt stem (Smith & Nephew, London, UK) is a monoblock TFT stem with a 3-degree distal taper (Image 1). Although the stem is available with modular proximal sleeves, none of the patients in our study had modular sleeves implanted. This monoblock stem was used in two international centers in 37 patients. Its use was non-randomized and according to surgeon preference. Ten patients had Paprosky grade 2 femoral bone loss, while the remainder of cases (n = 27) had grade 3A bone loss.
2.3 Modular stem group
The comparison group included rTHA cases in which a modular TFT stem was used (Restoration Modular, Styker, Mahwah, NJ) (Image 2). This stem utilizes a 2-degree distal taper. This modular stem was used in 27 patients according to surgeon preference. Seven patients had Paprosky grade 2 femoral bone loss, while the remainder of cases (n = 20) had grade 3A bone loss.
2.4 Surgery
Patients underwent general or spinal anaesthesia, received antibiotic chemoprophylaxis and surgery was performed through a posterior or direct lateral approach according to surgeon preference, with ETO used where necessary in order to facilitate removal of femoral components. If acetabular component revision was required it was undertaken using multi-hole porous titanium modular acetabular components. All surgeries were led by a surgeon with fellowship training in adult reconstruction. Supervised residents performed all or part of the procedure. Patients were permitted to weight bear immediately after surgery.
2.5 Evaluation of subsidence and bicortical contact
Anteroposterior plain radiographs were performed in the immediate post-operative period and at routine regular follow up. Radiographs were analysed by two independent observers for subsidence using a digital templating system (TraumaCAD, Brainlab). Radiographic calibration was performed using known component size: either the femoral head or the acetabular cup if the femoral head was not visible. Implant subsidence was determined by measuring the distance between a fixed point of the stem and the most proximal part of the lesser trochanter according to the system published by Callaghan et al.
17
Similar to prior studies reporting on subsidence, 10 mm was chosen as the cut off for significant subsidence.2
, 3
, 4
, 5
Bicortical contact is defined as the total length of cortical bone in contact with the femoral stem and was calculated on a single anteroposterior radiograph of the femur according to the system published by Moriarty et al.18
2.6 Statistical analysis
Summary statistics were calculated for patient variables and radiographic factors. To assess the distribution of the data the Shapiro-Wilk test, boxplots, histograms and QQ plots were utilized. When analyzing the normally distributed continuous variables, the Student t-test was employed, while for the non-parametric continuous variables the Mann-Whitney test was used. When comparing parametric data with non-parametric data, the Mann-Whitney test was also used. Due to low numbers in some categories (<5), Fisher's exact test was employed for categorical variables. Linear regression was used to determine a correlation between two continuous variables. Statistical significance was set at p ≤ 0.05. Inter-observer agreement was calculated using Cohen's kappa. Statistical analysis was performed using SPSS v25 (IBM Corporation, Armonk, NY).
3. Results
3.1 Demographics
In total 64 hips in 63 patients were included, with a mean age of 72.9 years (46-93) and a mean follow up of six months (193 days, range: 28-666) (Table 1). The monoblock (Redapt) group (n = 37) consisted of 20 females and 17 males with a mean age of 72.2. The modular (Restoration Modular) group (n = 27) consisted of 9 females and 18 males with a mean age of 73.6. Forty-nine patients were revised for aseptic loosening (monoblock group n = 29, modular group n = 20). Eleven patients were revised for peri-prosthetic fracture (monoblock group n = 4, modular group n = 7). Two patients in the monoblock group were revised for instability, one for failed femoral neck fracture fixation and one for infection. There was no difference in indication for revision between the two groups (p = 0.29). Eight patients in the monoblock group (21%) and six patients in the modular group (22%) underwent extended trochanteric osteotomy (ETO). 33 patients in the monoblock group and 24 patients in the modular group underwent concomitant acetabular revision. No re-revisions were performed.
Table 1Demographics, subsidence and bicortical contact (PJI: Prosthetic Joint Infection).
| Monoblock TFT (n = 37) | Modular TFT (n = 27) | |
|---|---|---|
| Age | 72.2 (Range 46-93) | 73.6 (Range 52-90) |
| Gender | ||
| Female | 20 (54.1%) | 9 (33.3%) |
| Male | 17 (45.9%) | 18 (66.7%) |
| Preoperative diagnosis | ||
| Aseptic loosening | 29 (78.4%) | 20 (74.1%) |
| Periprosthetic fracture | 4 (10.8%) | 7 (25.9%) |
| Instability | 2 (5.4%) | 0 (0%) |
| PJI | 1 (2.7%) | 0 (0%) |
| Conversion | 1 (2.7%) | 0 (0%) |
| Paprosky classification | ||
| I | 0 (0%) | 0 (0%) |
| II | 12 (32.4%) | 5 (18.5%) |
| IIIA | 24 (64.9%) | 19 (70.4%) |
| IIIB | 1 (2.7%) | 3 (11.1%) |
| IV | 0 (0%) | 0 (0%) |
| Acetabular revision | 33 (89.2%) | 24 (88.9%) |
| ETO | 8 (21%) | 6 (22%) |
| Subsidence | 2.13 mm | 3.15 mm |
| Subsidence >10 mm | 1 (2.7%) | 1 (3.7%) |
| Bicortical contact | 54.8 mm | 54.7 mm |
3.2 Subsidence
Interobserver reliability for radiographic assessment of subsidence was 0.74 and for bicortical contact was 0.7, indicating substantial agreement for both variables measured. There was no difference in subsidence between the two groups (p = 0.318). Mean subsidence was 2.13 mm in the monoblock group and 3.15 mm in the modular group. Two stems subsided >10 mm: one in the monoblock group and one in the modular group.
3.3 Bicortical contact
There was no difference in bicortical contact between groups (p = 0.98). Mean bicortical contact was 54.8 mm in the monoblock group and 54.7 mm in the modular group. There was a statistically significant linear correlation between subsidence and bicortical contact in the monoblock group (p = 0.044), however this was not noted in the modular group (p = 0.294).
4. Discussion
Regardless of the type of stem used in rTHA, one of the major concerns must be subsidence. In our study, there was no difference in subsidence between the two groups: mean subsidence was 2.13 mm in the monoblock group and 3.15 mm in the modular group. These rates of subsidence are comparable with recent literature on the same monoblock stem: Gabor et al. reported short-term outcomes and noted mean subsidence of 1.64 mm and 2% rate of subsidence >10 mm.
16
Ngu et al. reported on subsidence in 23 rTHAs at minimum four years post-op and noted only one case of subsidence >5 mm and no cases of subsidence >10 mm.19
In our study, two stems displayed significant subsidence (>10 mm), one in each group (3.1% overall). In both cases the cause of this subsidence was undersizing of the stem. On subsequent radiographs, settling of the stem and stability was noted in both cases.In our study there was no difference in bicortical contact between the stems. However, in the monoblock group, a statistically significant linear correlation between subsidence and bicortical contact was noted (p = 0.044), No correlation was found in the modular group. The monoblock stem has a 3-degree distal taper angle, with this taper aiming to provide stability over shorter cortical interfaces.
27
However our study would suggest that subsidence was still associated with inadequate bicortical contact, despite this taper angle. Similarly, Moriarty et al. compared outcomes of two modular cementless, tapered, fluted titanium stems and concluded that adequate stem length and bicortical contact had the most significant effect on subsidence, rather than minor differences in stem design and geometry.18
Patel et al. reported a single institution experience with distally fixed modular implants, and noted that all the stems requiring re-revision for subsidence were undersized.20
This highlights the importance of surgical technique to prevent subsidence by achieving adequate stem size and bicortical contact, regardless of the stem being used.While surgical technique is likely the most crucial variable in the success of these stems, implant design, specifically distal taper angle and spline geometry, remains a relatively unknown contribution to the stability of the stem. Pierson et al. noted that increasing taper angle and broader splines contributed significantly to axial stability.
21
The Redapt monoblock stem has a 3-degree distal taper angle. It employs a novel spline configuration, with tall major splines and broad minor spines, in an effort to further stabilize the stem. The Restoration Modular stem has uniform spline geometry surrounding the stem, with a 2-degree distal taper angle. Based on the rates of subsidence in our study, both designs achieved satisfactory early stability.In our study involving two modern TFT stems, there was no difference in subsidence between the stems evaluated, with low rates of post-operative subsidence, regardless of monoblock or modular design. The management of femoral bone loss in rTHA has evolved over time. Initially, fully coated cylindrical cobalt-chromium stems became popular because of high rates of success and relative ease of use.
22
However, the high modulus of elasticity of cobalt-chromium can cause stress shielding, proximal femoral bone loss and subsequent thigh pain.28
These concerns have caused fully coated cylindrical implants to become largely replaced with modular TFT stems. Modularity allows surgeons to recreate leg length and offset, improving stability compared to traditional monoblock stems.- Zhang Y.
- Zhang Y.
- Sun J.N.
- Hua Z.J.
- Chen X.Y.
- Feng S.
Comparison of cylindrical and tapered stem designs for femoral revision hip arthroplasty.
BMC Muscoskel Disord. 2020 Jun 29; 21 (PMID: 32600293; PMCID: PMC7325076): 411https://doi.org/10.1186/s12891-020-03461-5
12
However concerns exist regarding stem fracture at modular junctions.13
,23
,24
This risk of fracture at modular junctions causing catastrophic failure has led to renewed interest in monoblock stems, especially modern stems with options to recreate native anatomy. With the modern monoblock stem in our study, stem options allowing for accurate recreation of offset are available. This ability to recreate combined offset postoperatively is vital to reduce the risk of dislocation.
25
There are several limitations to our study. While the follow-up time in the study is relatively short, studies have shown that the majority of subsidence in cementless stems occurs within the first three months following surgery.
6
,16
,26
The study is retrospective in nature, and stem choice was based on surgeon preference, with no randomization process. However we present a pragmatic multicenter study, which provides important data on these stems when used by different surgeons for different indications.Managing femoral bone loss in rTHA is challenging, and the ideal femoral stem remains elusive. Early monoblock TFT stems displayed high rates of subsidence,
2
, 3
, 4
, 5
while modular TFT stems have a risk of implant fracture at modular junctions. The results of our study suggest that both stems designs display a low incidence of subsidence. In particular, the monoblock stem displayed superior rates when compared to earlier reports of similar stems, with no difference noted when compared to a modular design, in keeping with prior studies on the use of TFT stems.6
,28
,- Zhang Y.
- Zhang Y.
- Sun J.N.
- Hua Z.J.
- Chen X.Y.
- Feng S.
Comparison of cylindrical and tapered stem designs for femoral revision hip arthroplasty.
BMC Muscoskel Disord. 2020 Jun 29; 21 (PMID: 32600293; PMCID: PMC7325076): 411https://doi.org/10.1186/s12891-020-03461-5
29
Regardless of stem design, if using a TFT stem the most important factor to prevent subsidence is surgical technique, and this should be at the forefront of surgeon's minds; both of the stems which subsided in this study were found to be undersized. We suggest that surgeons consider the role of these monoblock stems in the setting or rTHA, as they have comparably low rates of subsidence and the use of monoblock stems, which eliminate modular junctions, removes the small but potentially serious complication of fracture at these modular junctions.- Yacovelli S.
- Ottaway J.
- Banerjee S.
- Courtney P.M.
Modern revision femoral stem designs have No difference in rates of subsidence.
J Arthroplasty. 2021 Jan; 36 (Epub 2020 Aug 5. PMID: 32863074): 268-273https://doi.org/10.1016/j.arth.2020.07.078
Author contributions
Eoghan Pomeroy: Participated in study design, data collection, writing and revising manuscript, Seán O. Flynn: Participated in study design, data collection, writing and revising manuscript, Mihai Grigoras: Participated in study design, statistical analysis, writing and revising manuscript, Terence P. Murphy: Participated in study design, writing and revising manuscript, Alexandra I. Stavrakis: Participated study design, data collection, writing and revising manuscript, Fiachra E. Rowan: Participated in study design, data collection, writing and revising manuscript.
Sources of financial support
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of competing interest
None.
References
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- Results of revision for mechanical failure after cemented total hip replacement, 1979 to 1982. A two to five-year follow-up.J Bone Joint Surg Am. 1985; 67: 1074-1085
- Bicortical contact predicts subsidence of modular tapered stems in revision total hip arthroplasty.J Arthroplasty. 2020; 35: 2195-2199
- Single 3 degrees tapered fluted femoral stems demonstrate low subsidence at mid-term follow-up in severe bony deficiency.Ann Transl Med. 2019; 7: 725
- Influence of technique with distally fixed modular stems in revision total hip arthroplasty.J Arthroplasty. 2010; 25: 926-931
- The effect of taper angle and spline geometry on the initial stability of tapered, splined modular titanium stems.J Arthroplasty. 2015; 30: 1254-1259
- 5- to 13-year follow-up study on cementless femoral components in revision surgery.J Arthroplasty. 1997; 12: 839-847
- A modular femoral implant for uncemented stem revision in THR.Int Orthop. 2000; 24: 134-138
- A modular distal fixation option for proximal bone loss in revision total hip arthroplasty: a 2- to 6-year follow-up study.J Arthroplasty. 2003; 18: 94-97
- Effect of restoration of combined offset on stability of large head THA.Hip Int. 2012; 22: 248-253
- The cone hip stem: a prospective study of 13 patients followed for 5 years with RSA.Acta Orthop Scand. 2003; 74: 525-530
- An in vitro comparison of the primary stability of 2 tapered fluted femoral stem designs.J Arthroplasty. 2016; 31: 517-523https://doi.org/10.1016/j.arth.2015.09.036
- Comparison of cylindrical and tapered stem designs for femoral revision hip arthroplasty.BMC Muscoskel Disord. 2020 Jun 29; 21 (PMID: 32600293; PMCID: PMC7325076): 411https://doi.org/10.1186/s12891-020-03461-5
- Modern revision femoral stem designs have No difference in rates of subsidence.J Arthroplasty. 2021 Jan; 36 (Epub 2020 Aug 5. PMID: 32863074): 268-273https://doi.org/10.1016/j.arth.2020.07.078
Article info
Publication history
Published online: September 12, 2022
Accepted:
September 7,
2022
Received in revised form:
August 20,
2022
Received:
June 25,
2022
Identification
Copyright
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