Does impaction matter in hip resurfacing? A cadaveric study

Influence of a Modified Procedure of Joining Ceramic Head and Adapter Sleeve on the Stem Taper in Revision: An Experimental Study

In revision operations, ceramic heads of modular hip implants can be replaced. As the surface of the stem taper can be damaged, additional adapter sleeves are applied. The components are usually connected manually by the surgeon in a one-step procedure by hammer impacts. In this study, we investigated a two-step joining procedure with reproducible impaction force. First, the adapter sleeve and head were joined quasi-statically with a force of 2 kN using an assembly device. In the second step, these components were applied to the stem taper using a pulse-controlled instrument. For reference, the joints were assembled according to standard conditions using a tensile testing machine. An average pull-off force of 1309 ± 201 N was achieved for the components joined by the instrument, and the average measured values for the components joined by the testing machine were 1290 ± 140 N. All specimens achieved a force >350 N when released and therefore met the acceptance criterion defined for this study. This study showed that a modified procedure in two steps with a defined force has a positive effect on the reproducibility of the measured joining forces compared to previous studies.

Periprosthetic fractures: concepts of biomechanical in vitro investigations

PURPOSE

Experimental in vitro studies investigating periprosthetic fractures after joint replacement are used increasingly. The purpose of this review was to deliver a condensed survey of studies in order to provide researchers with an overview of relevant scientific results and their clinical relevance.

METHODS

A literature search was conducted to obtain all available papers dealing with periprosthetic fractures, with particular attention being paid to articles with an experimental research design. Study goals, scientific methods and results, their interpretation and clinical relevance were assessed and compared. The main focus was on comparability with clinical fracture patterns and physiological joint loads.

RESULTS

Excluding duplicates, 24 studies with regard to artificial hip, knee and shoulder joints were found dating back to August 2000. Almost all studies were performed quasi-statically and without consideration of muscle forces and thus reflect selected loading conditions and no dynamic situation during activities of daily living (ADL). Various experimental protocols were used, differing in the choice of experimental material, implant and fixation system and load application.

CONCLUSIONS

In vitro studies regarding periprosthetic fracture research allow controlling for disturbances, such as clinically occurring risk factors like reduced bone mineral density (BMD) or greater patient age. Notwithstanding, due to methodological differences, comparisons between studies were possible to a limited degree only. For this reason, and because of quasi-static loading typically applied, results can only be partially applied to clinical practice.

Impact of implant size on cement filling in hip resurfacing arthroplasty

Larger proportions of cement within femoral resurfacing implants might result in thermal bone necrosis. We postulate that smaller components are filled with proportionally more cement, causing an elevated failure rate. A total of 19 femoral heads were fitted with polymeric replicas of ReCap (Biomet) resurfacing components fixed with low-viscosity cement. Two specimens were used for each even size between 40 and 56 mm and one for size 58 mm. All specimens were imaged with computed tomography, and the cement thickness and bone density were analyzed. The average cement mantle thickness was 2.63 mm and was not correlated with the implant size. However, specimen with low bone density had thicker cement mantles regardless of size. The average filling index was 36.65% and was correlated to both implant size and bone density. Smaller implants and specimens with lower bone density contained proportionally more cement than larger implants. According to a linear regression model, bone density but not implant size influenced cement thickness. However, both implant size and bone density had a significant impact on the filling index. Large proportions of cement within the resurfacing head have the potential to generate thermal bone necrosis and implant failure. When considering hip resurfacing in patients with a small femoral head and/or osteoporotic bone, extra care should be taken to avoid thermal bone necrosis, and alternative cementing techniques or even cementless implants should be considered. This study should help delimiting the indications for hip resurfacing and to choose an optimal cementing technique taking implant size into account.

Effect of cementing technique and cement type on thermal necrosis in hip resurfacing arthroplasty--a numerical study

Femoral fractures within resurfacing implants have been associated with bone necrosis, possibly resulting from heat generated by cement polymerization. The amount of heat generated depends on cement mantle volume and type of cement. Using finite element analysis, the effect of cement type and volume on thermal necrosis was analyzed. Based on CT-data of earlier implantations, two different models were created: a thick mantle model, representing a low-viscosity "cement filling" technique, and a thin mantle model, representing a high viscosity "cement packing" technique. Six cement types were analyzed. The polymerization heat generation and its effect on bone necrosis were predicted. In the thin cement mantle models, no thermal necrosis was predicted. Thick cement mantle models produced thermal necrosis at the cement-bone interface depending on cement type. In the worst case, 6% of the bone at the cement-bone interface became necrotic, covering almost the entire cross-sectional area. The current findings suggest a potential thermal drawback of thick cement mantles, although it is unclear whether thermal bone necrosis significantly affects implant fixation or increases the fracture risk. Furthermore, our study showed distinct differences between the heat generated and resulting thermal damage caused by the various cement types.

Influence of assembly procedure and material combination on the strength of the taper connection at the head-neck junction of modular hip endoprostheses

BACKGROUND

A stable fixation between femoral head and endoprosthesis taper is necessary to prevent relative motions and corrosion at the taper junction. Although the importance of the component assembly has been recognised, no definitive instructions are available. The purpose of this study was to assess the influence of assembly force, assembly tool and number of hammer strokes on the taper junction strength of various material combinations.

METHODS

Co-Cr29-Mo (n=10) and Ti-6Al-4V (n=10) neck tapers were assembled with Co-Cr and Al(2)O(3) ceramic heads either by push-on or by impaction with single or multiple hammer blows. The strength of the taper-head connection was evaluated by measuring the head pull-off forces according to ISO 7206-10 and the turn-off moment capacity.

FINDINGS

The taper strength linearly increased with assembly forces (P<0.001). Co-Cr heads combined with Co-Cr tapers showed significantly lower pull-off forces and turn-off moments than the combination with Ti tapers (0.001<P<0.025). Multiple impaction did not increase taper strength (0.063 <P<0.995). Ceramic and Co-Cr heads showed similar fixation patterns on Ti tapers. Turn-off moments varied between 6 Nm and 19 Nm, dependent on material combination and assembly force.

INTERPRETATION

It is suggested that sufficient head-taper junction strength in all bearing conditions is achieved by impaction forces of at least 4 kN. A single impact is sufficient to achieve fixation. Special attention should be paid to the assembly of Co-Cr heads on Co-Cr tapers.

Effects of impaction on cement mantle and trabecular bone in hip resurfacing

INTRODUCTION

Failures involving the femoral part in hip resurfacing remain problematic in terms of overall implant survival. In this context, effects of impaction strength on cement mantle morphology and trabecular bone damage have not been studied to date.

MATERIALS AND METHODS

Sixteen paired cadaveric femora that had undergone hard and gently impacted hip resurfacing using a manual packing cementing technique in a previous study, were evaluated morphologically. The earlier study had revealed lower failure loads for hard impacted heads. A central slice of each femoral head underwent microradiography.

RESULTS

Overall cement mantle thickness averaged 2.0 mm (range 0-5 mm) in the hard and in the low impact group with no significant difference between groups (p = 0.299). No signs of damage in the bone remnants inside the prosthesis of the fractured proximal femurs were detected in the microradiographic analysis.

CONCLUSION

Cement mantle thickness was not influenced by impaction strength when using a manual packing cementing technique. No trabecular damage underneath the implant was detected despite lower failure loads, confirming the difficulty to identify small trabecular damage in an in vitro study.

Complications after metal-on-metal hip resurfacing arthroplasty

This article determines the incidence and cause of the complications commonly associated with metal-on-metal hip resurfacing implants and the proposed methods to prevent these complications. The literature available in PubMed was reviewed. Complication rates after hip resurfacing are low, and the procedure has shown both safety and efficacy in the hands of surgeons trained in specialized centers. Proper surgical technique can further reduce the incidence of femoral neck fracture, component loosening, and abnormal wear of the prosthesis. A more systematic detection of adverse local tissue reactions is needed to provide accurate assessments of their prevalence.

Biomechanics of short hip endoprostheses--the risk of bone failure increases with decreasing implant size

BACKGROUND

Short uncemented metaphyseally anchored femoral endoprostheses are becoming popular and are proposed to be less invasive than longer conventional implants. However, it is proposed here that shortening femoral endoprostheses can increase the risk of periprosthestic fracture.

METHODS

A simple analytical model of a femoral hip implant was developed to estimate the risk of bone overload for varying implant size, implantation geometry, implantation force and bone quality. The load capacity of a particular short implant design in poor quality cadaveric bone specimens was also measured experimentally, to validate the model.

FINDINGS

The model demonstrated a high risk of bone overload for a short endoprosthesis in poor quality bone. The experimental results and a clinical example of failure, to which the model was applied, supported this finding. Bone stresses increased with decreasing implant length and diameter, varus implantation, incomplete seating and high implantation forces, approaching the strength of good quality bone in extreme cases.

INTERPRETATION

Correct implantation and patient selection is essential for short femoral endoprostheses.