Fixation of non-cemented total hip arthroplasty femoral components in a simulated proximal bone defect model

The insidious risk of periprosthetic fracture in clinically functional total hip arthroplasties: A biomechanical study of willed joints

Femoral bone quality is a major risk factor of periprosthetic fracture after total hip arthroplasty (THA), which has mortality similar to native hip fractures but higher short-term morbidity. The goal of this study was to quantify cortical strains at the site of expected Vancouver Type-B periprosthetic fracture as a function of bone mineral density, femoral stem material, and fixation method using a series of 29 autopsy-retrieved, clinically asymptomatic hip joints with THA. Periprosthetic bone mineral content and density was assessed using dual-energy X-ray absorptiometry by Gruen Zone. Specimens then underwent combined cyclic axial and torsional loading, increasing incrementally from 100 N and ±1 Nm to peaks of 700 N and ±5 Nm. All specimens experienced significantly higher strains on the lateral surface than on the anterior surface, indicating that the bending loads in the frontal plane, rather than axial/torsional loads, had the predominant effect. Multiple significant relationships (p = 0.04, p = 0.02) were found between predicted periprosthetic strains calculated from radiographic measurements and observed principal strains. Though THA in the present study were in successful clinical service, the produced results indicated that some femurs with rigid cemented or noncemented implants were potentially at high risk for Vancouver Type-B fractures, which may be predicted radiographically.

Primary stability of a cementless modular revision hip stem in relation with the femoral defect size: A biomechanical study

PURPOSE

Cementless modular fluted hip stems are commonly used in revision arthroplasty. Nevertheless, there is a wide spectrum of recommendations concerning the minimum bone stock required to enable osseous ingrowth and implant-bone micromotions <100 µm. This experimental study investigated the primary stability of a tapered cementless fluted revision stem depending on different types of bone defects.

METHODS

Implant-bone interface movements with a bimodular stem were examined under cyclic axial and torsional loading using composite femora. In four degrees of freedom, the implant subsidence and micromotions were captured with linear variable differential transformers for the intact femora and seven different defects ranging from Paprosky type I to type IIIB.

RESULTS

With a 7-cm length of intact diaphysis proximal to the isthmus (Paprosky IIIA), mean implant-bone micromotions of 66 µm occurred. An implant-bone contact zone of only 5 cm (Paprosky IIIA) resulted in micromotions notably over 100 µm and significantly increased subsidence (p < 0.05). With a Paprosky IIIB defect (3 cm of intact diaphysis) rotational instability occurred in all specimens.

CONCLUSIONS

Aside from critically increased interfacial micromotions (>100 µm), rotational instability emerged as a mechanism of fixation failure when the implant-bone contact zone was only 5 cm or less. Hence, future studies investigating the implant fixation in the case of femoral bone defects should consider both axial and torsional loading. With regard to the clinical application, our data suggest maintaining 7 cm of diaphyseal implant-bone contact for a safe anchorage of cementless fluted hip revision stems.

Characterization of Femoral Component Initial Stability and Cortical Strain in a Reduced Stem-Length Design

BACKGROUND

Short-stemmed femoral components facilitate reduced exposure surgical techniques while preserving native bone. A clinically successful stem should ideally reduce risk for stress shielding while maintaining adequate primary stability for biological fixation. We asked (1) how stem-length changes cortical strain distribution in the proximal femur in a fit-and-fill geometry and (2) if short-stemmed components exhibit primary stability on par with clinically successful designs.

METHODS

Cortical strain was assessed via digital image correlation in composite femurs implanted with long, medium, and short metaphyseal fit-and-fill stem designs in a single-leg stance loading model. Strain was compared to a loaded, unimplanted femur. Bone-implant micromotion was then compared with reduced lateral shoulder short stem and short tapered-wedge designs in cyclic axial and torsional testing.

RESULTS

Femurs implanted with short-stemmed components exhibited cortical strain response most closely matching that of the intact femur model, theoretically reducing the potential for proximal stress shielding. In micromotion testing, no difference in primary stability was observed as a function of reduced stem length within the same component design.

CONCLUSION

Our findings demonstrate that within this fit-and-fill stem design, reduction in stem length improved proximal cortical strain distribution and maintained axial and torsional stability on par with other stem designs in a composite femur model. Short-stemmed implants may accommodate less invasive surgical techniques while facilitating more physiological femoral loading without sacrificing primary implant stability.

Tapered vs Cylindrical Stem Fixation in a Model of Femoral Bone Deficiency in Revision Total Hip Arthroplasty

BACKGROUND

Distal fixation achieved with a tapered stem design has demonstrated favorable clinical results in revision total hip arthroplasty in the setting of severe bone defects. However, stem subsidence is common with this stem design.

PURPOSE

The purpose of this study is to compare the initial fixation stability of a tapered stem design to a fully porous-coated cylindrical stem design in a model of severe femoral bone deficiency.

METHODS

Tapered and cylindrical stems (n = 8) were implanted into a model femur with progressively shorter segments for fixation (9, 6, or 3 cm). The stems were axially loaded, and the force to produce subsidence was recorded.

RESULTS

Average loads to produce 150 μm of displacement with a 3-cm segment were higher for the tapered stem (393 N vs 221 N, P < .01). No difference was observed in the 6- or 9-cm models. Average loads to produce failure (>4-mm subsidence) were also higher for tapered stems with a 3-cm segment (1574 N vs 500 N, P < .0001). A regression analysis determined the minimum segment length of 1.5-2.5 cm to obtain stable fixation with a tapered stem design (R(2) = 0.78, P < .001).

CONCLUSIONS

Tapered stems required higher loads to produce subsidence than cylindrical stems in a revision THA model. Revision tapered stems require a minimum intact segment of 1.5-2.5 cm to obtain adequate initial fixation stability. Revision tapered stems have superior initial fixation stability to cylindrical stems in the setting of severe bone loss.

An In Vitro Comparison of the Primary Stability of 2 Tapered Fluted Femoral Stem Designs

BACKGROUND

Proximal bony deficiencies present a biomechanical challenge to achieving primary stability in revision hip arthroplasty. Long tapered fluted stems have been engineered to span these defects but concerns of early subsidence are well documented. This work aimed primarily to investigate the issue of subsidence with this design using a cadaveric model. A secondary aim was to compare the stability of 2 versions of this design.

METHODS

Seven pairs of cadaveric femora were obtained, dual emission x-ray absorpitometry scanned, with calibration radiographs taken for digital templating. Each bone was potted according to the ISO standard for fatigue testing and a Paprosky type 3 defect was simulated. The established cone-conical Restoration Modular (Stryker) system and a novel design with a chamfered tip and flute configuration (Redapt, Smith & Nephew) were examined. Movement at the stem-bone interface was measured using radiostereometric analysis and micromotion transducers.

RESULTS

All restoration stems and 85% of the Redapt stems achieved stability by recognized criteria, micromotion < 150 μm and migration less than 2 mm. A Fisher exact test comparing the proportion of stems which were stable or unstable was not significant, P = .055. Mean axial subsidence (SD) was 0.17 mm (0.32) and 0.1 mm (0.131) for the Restoration and Redapt stems respectively.

CONCLUSION

This study has demonstrated minimal subsidence in the immediate post-operative period using tapered fluted stems. Both designs achieved excellent stability despite simulation of Paprosky type 3 bony defects in the cadaveric model. This geometry appears satisfactory for use in revision surgery in the presence of significant proximal bony deficiencies.

Outcomes of Mixed Femoral Fixation Technique Using Both Cement and Ingrowth in Revision Total Hip Arthroplasty: Minimum 2-Year Follow-up

The use of a modular femoral stem in revision total hip arthroplasty (THA) has been increasing recently. However, complications such as subsidence, dislocation and stem fracture are still noted, especially in hips with high grade femoral deficiency. We retrospectively studied a consecutive 41 hips (40 patients) that underwent revision THA with allograft reconstruction of the proximal femur in conjunction with hybrid fixation (proximally cemented and distally press-fit) of a modular femoral component. At a mean follow-up of 5.2 years (2 to 8 years), no hips sustained dislocation, subsidence or fracture of the stem in the follow-up period. We provided evidence that this technique may be a good alternative in the management of proximal femoral bone loss during revision THA.

Management of severe femoral bone loss in revision total hip arthroplasty

Femoral bone loss is a complex problem in revision total hip arthroplasty. The Paprosky classification is used when determining the degree and location of bone loss. Meticulous operative planning is essential where severe bone loss is a concern. One must correctly identify the bone loss pattern, safely remove the existing components, and proceed with the proper reconstruction technique based on the pattern of bone loss. This article discusses the etiology and classification of bone loss, clinical and radiographic evaluation, components of effective preoperative planning, and clinical results of various treatment options with a focus on more severe bone loss patterns.