Which length should the neck segment of modular revision stems have?

Femoral revision in total hip arthroplasty using a cementless modular stem: clinical and radiological results with a 8-year follow-up

INTRODUCTION

Modular femoral components allow for patient-specific restoration of hip joint geometry and the reconstruction of extensive bone defects in revision total hip arthroplasty (THA); however, potential problems of modular implants such as taper corrosion and the risk of implant fracture continue to be of concern. The aim of the present study was to evaluate the clinical and radiological results of a cementless modular revision stem following revision surgery due to aseptic loosening and periprosthetic fracture and to assess patient-reported outcome measures (PROMs) in these patients at mid-term follow-up.

MATERIALS AND METHODS

In this study, a consecutive cohort of 75 patients who underwent primary revision THA at our institution using a modular cementless stem design (MRP-TITAN stem) was retrospectively evaluated at a mean follow-up of 7.7 years. Kaplan-Meier survivorship analyses were performed with revision of the femoral component for any reason as the end point. The Harris-Hip Score, the UCLA Activity Score, the Forgotten Joint Score and the SF-12 Score were used for clinical assessment. We used the Wilcoxon signed rank test to compare pre- and postoperative clinical scores.

RESULTS

Overall stem survival with the endpoint stem re-revision for any reason was 85.4% at a mean follow-up of 7.7 years (range 2.4-14 years). Stem survival was 89.5% in the aseptic loosening group and 78.3% in the periprosthetic fracture group with no statistically significant difference between both groups (p = 0.107). One patient had to be revised due to taper fracture. PROMs improved significantly up to the latest follow-up, and radiographic evaluation showed full osseointegration of all stems in this cohort.

CONCLUSIONS

Revision THA using a modular cementless titanium revision stem demonstrated adequate clinical and radiological results at mid- to long-term follow-up in this cohort. Cementless revision stems are a useful treatment option to restore the anatomy, especially in deformed hips and in complex revision hip arthroplasty. However, there are some significant disadvantages related to an increased risk of mechanical failure such as corrosion/fretting damage and implant fracture. Future high-quality prospective studies with longer follow-up are necessary to confirm the supposed advantages.

[Strength of the taper junction of modular revision hip stems : The influence of contamination of the taper junction and the joining condition on the relative motion and pull-off force between the stem and neck part of the MRP prosthesis]

INTRODUCTION

In revision surgery, modular implant components allow the surgeon to tailor the characteristics of the implant to the bone situation. Relative motion can occur at the tapered modular connection, leading to fretting corrosion and subsequent biological reactions, particularly due to poor assembly and contamination of the tapered connection. The aim of this study was to demonstrate whether incomplete assembly and inadvertent contamination of the modular taper causes a change in junction strength.

MATERIAL AND METHODS

Modular taper junctions between the neck and the stem (n = 48) were divided into seven groups that differed with respect to contamination (native, contaminated, cleaned) and assembly conditions (secured, pre-tensioned and secured). Contamination was achieved by a combination of porcine bone particles and bovine blood. For each group, the number of rotations of the torque limiter while securing the conical connection was recorded. The implants were subjected to cyclic loading. DIC was used to determine neck rotation, micromotion and axial subsidence. Loosening torque of the locking screw and pull-off forces were measured as an equivalent of residual taper junction strength.

RESULTS

Contamination of the taper junction, especially in combination with improper assembly of the components, significantly increased the rotation (35.3 ± 13.7° vs. 2.4 ± 4.4°; p <0.001), micromotion (67.8 ± 16.9 μm vs. 5.1 ± 12.1 μm, p <0.001) and axial subsidence (‑34.1 ± 16.9 μm vs. 4.3 ± 10.9 μm; p <0.001) of the neck relative to the stem.

CONCLUSION

Intra-operatively, contamination of the taper surface can be identified by the need for multiple turns when tightening the locking screw. Correct cleaning with the new taper cleaning instrument and complete assembly with pre-tensioning may reduce the risk of early failure and fatigue fracture of the modular taper connection.

The influence of hip revision stem spline design on the torsional stability in the presence of major proximal bone defects

BACKGROUND

Despite the success of primary total hip arthroplasty, the number of revisions remains high. Infection, aseptic loosening, periprosthetic fractures and dislocations are the leading causes of hip revision. Current revision stem designs feature a tapered body with circumferential placed longitudinal thin metal splines that cut into the femoral cortex of the diaphysis to provide axial and rotational stability. Modifications to the spline design may help improve primary stability in various bone qualities. The purpose of this study was to evaluate whether the rotational stability of a revision hip stem can be improved by an additional set of less prominent, wider splines in addition to the existing set of splines. It is hypothesized that the additional splines will result in greater cortical contact, thereby improving torsional strength.

METHODS AND FINDINGS

The ultimate torsional strength of an established modular revision stem (Reclaim®, DePuy Synthes) was compared to a Prototype stem design with two sets of splines, differing in prominence by 0.25 mm. Five pairs of fresh-frozen human femurs (n = 10) were harvested and an extended trochanteric osteotomy was performed to obtain common bone defects in revision. Stems were implanted using successive droptower impacts to omit variability caused by mallet blows. The applied energy was increased from 2 J in 1 J increments until the planned implantation depth was reached or seating was less than 0.5 mm at 5 J impact. The ultimate torsional strength of the bone-to-implant interface was determined immediately after implantation. Image superposition was used to analyze and quantify the contact situation between bone and implant within the femoral canal. Cortical contact was larger for the Prototype design with the additional set of splines compared to the Reclaim stem (p = 0.046), associated with a higher torsional stability (35.2 ± 6.0 Nm vs. 28.2 ± 3.5 Nm, p = 0.039).

CONCLUSIONS

A second set of splines with reduced prominence could be shown to improve primary stability of a revision stem in the femoral diaphysis in the presence of significant proximal bone loss. The beneficial effect of varying spline size and number has the potential to further improve the longevity of revision hip stems.

Breakage of Tapered Junctions of Modular Stems in Revision Total Hip Arthroplasty—High Incidence in a Consecutive Series of a Single Institution

Background: Modularity in revision THA (RTHA) has become accepted during the last three decades. Nevertheless, specific risks of modularity of current revision devices such as breakage of taper junctions occur during follow-up. Data reporting failure rates are predominantly given by the manufacturers but independent data acquisition is missing so far. Questions/Purposes: 1. What time-related risk of breakage of taper junction between neck and body of an established modular revision device can be expected in a consecutive single institutional series and a mid-term follow-up? 2. Are there specific factors influencing breakage in this cohort? Materials and Methods: A retrospective analysis was performed of a consecutive series of 89 cases after femoral revision using a tapered modular revision stem. Mean follow-up period was 7.1 (range: 3.0–13.7) years. Breakage of stem as failure criteria of the implant was investigated with a Kaplan–Meier analysis. Results: Breakage of taper junctions occurred in four patients during follow-up showing a time-depending implant survival of 94.2 (95% CI: 88.6–100%) after 13.7 years. Implant survival of stems with lateralized necks of 87.4 (95% CI: 75.6–100%) after 13.7 years was significantly lower compared to the standard offset variant with 100% after 13.5 years (log rank test p = 0.0283). Chi square test also revealed a significantly higher risk of breakage of lateralized necks compared to standard offset pieces (p = 0.0141). Three of four patients were obese with a mean BMI of 37.9 kg/m2. Grade of obesity (grade 1 or higher) had significant influence on risk of breakage. Survival of the implant was significantly lower in obese patients with at least grade 1 obesity compared to patients with a BMI < 30 kg/m2 (82.9 (95% CI: 64.9–100%) after 11.6 years vs. 98.4 (95% CI: 95.3–100%) after 13.7 years; log-rank p = 0.0327). Conclusions: Cumulative risk for failure of taper junctions was high in this consecutive single institutional cohort and may further increase during follow-up. As independent data acquisition in registries is missing, failure rate may be higher than reported data of the manufacturers. The use of lateralized offset necks in obese patients of at least grade 1 obesity showed a significantly higher risk of breakage. The use of monobloc revision devices may be an option, but randomized control trials are currently missing to establish standardized treatment protocols considering individual risks for both monobloc and/or modular implants.

[Pitfalls in revision hip arthroplasty]

Significant key points for a successful revision surgery in hip arthroplasty are as follows: identification of the implants, preoperative clinical diagnoses, planning (including alternatives) as well as establishing a surgical strategy, selection of revision implants and provision with special tools, secure fixation of acetabular and femoral components and the reconstruction of a stable joint without risk of dislocation or impingement. It seems to be obvious that most problems can be avoided by clear and concise decision-making. Some tips and tricks are also presented.

[Strategies for stem revision : Surgery planning, implant removal and reimplantation]

BACKGROUND AND PLANNING

Femoral revision represents a challenging procedure in its surgical planning and technical execution. The results are mainly dependent on the bone loss present, the quality of the implant removal and the choice of revision system. Patient-specific factors such as age, comorbidities, bone quality or even the localization of the cement have a decisive influence on the surgical strategy. Appropriate preoperative preparation, including the presence of the necessary special instruments, which are essential for gentle implant removal, is indispensable to achieve the best possible result.

THERAPY

A bone-sparing implant removal can be particularly challenging, especially in the case of well-fixed stems and cement residues, which may extend far beyond the isthmus. In such situations, a transfemoral approach should be considered. Cementless reimplantation using modular or non-modular titanium stems is currently the preferred treatment of choice for most revisions. In elderly, low-demand patients or with poor bone quality, cemented revision remains a good alternative treatment option.

Influence of bone morphology and femur preparation method on the primary stability of hip revision stems

Aseptic loosening is one of the major reasons for re-revisions of cementless revision stems. Insufficient primary stability is associated with bone characteristics and the surgical process. This study aimed to investigate how femur morphology and preparation methods influence the primary stability of revision stems. The Femur morphology was described by the upper femoral curvature (UFC) and an individualized Dorr type classification based on the ratio between the canal-to-calcar ratio (CCR*) and the cortical index (CI*) introduced as the cortical-canal shape (CCS). Manual and powered reaming in combination with helical and straight reamers were used to prepare the bone cavity of 10 cadaveric human femur pairs. Forces during stem impaction were recorded (Reclaim, Depuy Synthes). Micromotion at the bone-implant interface during cyclic axial loading and torsional load to failure was determined. The CCS and impaction forces (R²  = 0.817, p < 0.001) or torsional strength (R²  = 0.577, p < 0.001) are inversely related. CCS did not correlate with micromotion during axial loading (R²  = 0.001, p > 0.999), but proximal femoral curvature did (R²  = 0.462, p = 0.015). Powered reaming and straight reamers led to an improved torsional strength (both: p = 0.043). The Individualized Dorr classification CCS and UFC allows a good estimation of the primary stability of revision stems. For severely curved Dorr type-C femurs, an alternative anchorage method should be considered clinically.

The different failure modes of the connecting elements of the modular hip arthroplasty revision stem Revitan

INTRODUCTION

Fracture of the femoral stem is the cause of ~1 % of revisions after total hip arthroplasty. The risk increases intrinsically with modularity, whereby modularity is particularly useful in revision arthroplasty. We present 7 cases of failure of a specific modular, tapered, fluted, titanium alloy Revitan stem and analyse the different failure modes.

METHODS

Retrospective review of all Revitan stems revised at our institutions due to implant failure and analysis of clinical presentation, diagnostic workup, and failure mode. The retrieved components were analyzed by optical and scanning electron microscopy.

RESULTS

A total of 7 cases were included. There was a significant time lag between symptom onset and correct diagnosis. Conventional radiographs and low-dose CT scans (CT scout imaging) were decisive for diagnosis. All failures occurred at the level of the connection between the proximal component and the distal part of the stem. Three different failure modes were identified: loosening of the proximal component, fatigue fracture of the connection pin, and distal loosening of the connection pin. No alterations of the microstructure or deviation from manufacturing specifications regarding dimensions were observed. Failure was caused by mechanical overload.

CONCLUSION

Conventional radiographs are the mainstay in identifying failed modular stems. Repeated radiographs and low-dose CT scans may be helpful additions. No single modification of the connection will address all possible failure modes. Modularity of revision stems offers advantages up until insertion of the definitive stem. Monoblock definitive stems might overcome the potential mechanical weaknesses of modularity and should be considered in relatively young, heavy and active patients.