Initial stability comparison of modular hip implants in synthetic femurs

In vitro assessments of reverse glenoid stability using displacement gages are misleading - recommendations for accurate measurements of interface micromotion

BACKGROUND

Baseplate micromotion of the reverse shoulder glenoid component can lead to implant loosening. We hypothesized that a remotely positioned displacement gage measures elastic deformation of the system rather than actual micromotion at the implant/bone interface.

METHODS

Reverse glenoid components were implanted into polyurethane blocks of 3 different densities. A 700 N compressive load was maintained and a vertical 700 N shear load was applied for 1000 cycles. In addition to the typical gage measurement, a digital image analysis of micromotion at the implant/block interface using high resolution cameras was performed. The measurements were validated on human specimens. A finite element model was implemented to study the isolated effect of block deformation on baseplate displacements.

FINDINGS

With the gage, typically reported micromotions were measured. Two orders of magnitude lower micromotions were detected using interface image-based analysis. The finite element simulation showed that elastic deformation alone can cause micromotion magnitudes as measured with displacement gages. Polyurethane blocks of 20 and 15 lbs per cubic foot density best reproduced micromotions as measured on human specimens.

INTERPRETATION

We found considerably less relative micromotion at the implant/bone interface than previously assumed. Gage measurements quantify elastic deformation and not true interface micromotion. High resolution digital imaging at the implant/bone interface is strongly recommended for an accurate assessment of reverse glenoid component micromotion. Tests should further adopt 20 or 15 pcf bone test surrogates. Further studies are required to identify the failure modes encountered in vivo, and a corresponding in vitro testing methodology can then be developed.

Total hip arthroplasty using S-ROM prosthesis for dysplastic hip

PURPOSE

The purpose of this study was to evaluate the clinical and radiological results of total hip arthroplasty using a proximal modular femoral stem in patients who had secondary coxarthrosis associated with a dysplastic hip.

MATERIALS AND METHODS

Forty-two patients (45 hips) with secondary coxarthrosis were evaluated after undergoing primary total hip arthroplasty using an S-ROM proximal modular femoral stem. The average follow-up was 80 months (range: 60 to 96 months). Clinical and radiological assessments were performed based on the Harris hip score and the radiological changes around the prosthesis.

RESULTS

The average Harris hip score improved from 52.2 points to 88.5 points. All femoral stems showed stable fixation; there were 37 cases by bony ingrowth and 8 cases by stable fibrous ingrowth. Neither osteolysis nor progressive radiolucent lines around the femoral stem were found at the last follow-up. Forty-one hips (91.9%) revealed excellent or good clinical results at the most recent follow-up.

CONCLUSION

For advanced secondary coxarthrosis, total hip arthroplasty with the use of the proximal modular femoral stem yielded good mid-term results with respect to the clinical and radiological criteria.

The effect of screw position on the initial fixation of a reverse total shoulder prosthesis in a glenoid with a cavitary bone defect

Patients with rotator cuff tear arthropathy can be treated successfully with a reverse total shoulder prosthesis. In patients with significant glenoid bone loss, achieving stable bone fixation can be challenging, as the surgeon must know when bone grafting is necessary and when the plan to implant the reverse total shoulder prosthesis should be abandoned because of the likelihood of early implant loosening. The purposes of this study were (1) to determine the initial stability of a metal glenoid implant fixed in a glenoid with a central cavitary defect and (2) to determine whether an altered screw configuration would sufficiently resist implant micromotion and, thereby, allow bone ingrowth to occur. The Delta III reverse total shoulder glenoid implant was fixed into foam scapulae with a uniform density similar to normal glenoid bone density. The control group implants were fixed into foam scapulae without a glenoid defect, by use of the standard surgical technique for screw placement. The second group was fixed into foam scapulae containing a central cavitary glenoid defect, by use of the standard surgical technique for screw placement. The central cavitary defect was meant to simulate the bone loss typically found after the removal of a loose pegged glenoid implant, and it was created with a 4-pegged glenoid drill guide. A third group was fixed into foam scapula with a central cavitary glenoid defect, with an experimental screw configuration by use of a posterior screw directed toward the spine of the scapula and an anterior screw directed inferior to the central peg. All specimens were loaded with 500 cycles of 1 body weight (70 kg) to simulate the forces generated during arm elevation that occur during the first 3 months after surgery. Micromotion between the implant and the foam bone was measured with a digital video motion analysis system (accuracy, +/- 2.6 microm). After loading of the implant with 70 kg for 500 cycles in the superior direction, the mean micromotion was 54 microm (SD, 22) in the control group, 159 microm (SD, 70) in the second group, and 86 microm (SD, 32) in the third group (P = 0.003). Fixing the posterior screw into the spine of the scapula and directing the anterior screw below the central peg decreased the micromotion of a metal glenoid implant fixed in a glenoid with a cavitary defect by 46% and, more importantly, reduced the micromotion below the critical threshold of 150 microm, which is necessary for bone ingrowth and long-term survival of the implant.

Fatigue performance of composite analogue femur constructs under high activity loading

Synthetic mechanical analogue bone models are valuable tools for consistent analysis of implant performance in both equilibrium and fatigue biomechanical testing. Use of these models has previously been limited by the poor fatigue performance when tested under realistic service loads. An objective was to determine whether a new analogue bone model (Fourth-Generation) using enhanced analogue cortical bone provides significantly improved resistance to high load fracture and fatigue as compared to the current (Third-Generation) bone models in clinically relevant in situ type testing of total hip implants. Six Third-Generation and six Fourth-Generation mechanical analogue proximal femur models were implanted with a cemented mock hip arthroplasty. Each specimen was loaded at 5 Hz in simulated one-legged stance under load control with a maximum compressive load of 2670 N and load ratio of 0.1. Average complete structural failure in Third-Generation femurs occurred at 3.16 million cycles; all specimens exhibited substantial displacement and crazing at well below 3 million cycles. In contrast, all Fourth-Generation femurs sustained 10 million cycles without complete structural failure and showed little change in actuator deflection. The Fourth-Generation femur model performance was sufficient to allow the model to be used in biomechanically relevant load bearing levels with an intramedullary device without model compromise that would affect test results.

The S-ROM cementless femoral stem: history and literature review

The S-ROM femoral prosthesis, building on the foundation of the Sivash Stem, pioneered proximal modularity in total hip arthroplasty. Now with a clinical history dating back >20 years, the S-ROM has performed well in the primary and revision setting. Early concerns regarding fretting and failure at the modular junction have been overcome by basic science data and uncomplicated mid- to long-term clinical follow-up. Coupled with its clinical performance, the versatility of the stem has enabled the S-ROM to achieve a well-deserved prominence for use in complex and straightforward primary and revision total hip arthroplasty.

A study on the effect of dual blasting with TiO2 on titanium implant surfaces on functional attachment in bone

In the present study, the effect of a dual treatment of titanium implants and the subsequent bone response after implantation were investigated. Coin-shaped c.p. titanium implants were placed into the tibias of 12 rabbits. The implant, which was dually blasted with TiO2 particles of two different sizes, was compared with implants that were blasted with only one of these particle sizes. Implants in group 1 were grit blasted with small particles, 22-28 microm in size, and group 2 with coarser particles, 180-220 microm size. These two treatments gave different surface micro textures. To test the effect of a combination of two different treatments, group 3 implants were blasted first with the 180- to 220-microm and subsequently with the 22- to 28-microm particles. The surface topography of the implant specimens was examined by scanning electron microscopy and by a confocal laser scanner and a numeric evaluation of S(a), S(t), and S(dr) was recorded. Group 2 implants, which were blasted with only the coarse particles, showed a significantly better functional attachment (p < 0.001) than the other two groups. Group 1, which was blasted with only small particles, showed the lowest retention in bone. There was a positive correlation between the topographical and mechanical evaluation of the surfaces.

The primary stability of a cementless stem varies between subjects as much as between activities

The rehabilitation program adopted immediately after a cementless total hip replacement is a very important factor, because of the known relationship between osseointegration and implant micromotion. The present study was aimed to evaluate which type of task is the most critical in terms of bone-implant relative micromotion. Both inter-task and inter-subject variability were taken into account to verify if the movement strategy could be determinant on this assessment. A previously validated finite element model was used to predict the peak total micromovements over the entire bone-implant contact surface in four different patients, performing nine different tasks, using published data on joint forces recorded by instrumented hip prostheses. The results predicted by the various simulations suggest that while stair climbing is surely a critical task for primary stability, for some subjects other tasks may be as critical as stair climbing. From a variance analysis for simple crossover design on the predicted peak micromotion, the inter-subject variability had much more influence on the primary stability of cementless implant than the inter-task variability. Even if the results of Patient IBL, who was reported to have difficulties to perform any activities in a normal way, were excluded from the statistical analysis, the inter-subject variability remained still higher than the inter-task variability. The results obtained from simulations suggest that the strategy the hip replacement patient adopts to perform a given motor task, may be, for the implant stability, equally or even more critical than the type of motor task performed.