Effect of bearing friction torques on the primary stability of press-fit acetabular cups: A novel in vitro method

The Use of Porous Titanium Coating and the Largest Possible Head Do Not Affect Early Cup Fixation: A 2-Year Report from a Randomized Controlled Trial

Cups are more frequently revised than stems after uncemented total hip arthroplasty, which warrants the development of cup surfaces that provide long-lasting, stable fixation. Large heads have become popular with the aim of reducing dislocation rates, but they generate greater frictional torque that may compromise cup fixation. We aimed to investigate (1) if a novel porous titanium surface provides superior cup fixation when compared with a porous plasma spray (PPS) surface and (2) if the use of the largest possible head compromises cup fixation when compared with a 32-mm head.

METHODS

Ninety-six patients were randomized to receive either a cup with a porous titanium coating (PTC) or a cup with PPS. A second randomization was performed to either the largest possible (36 to 44-mm) or a 32-mm head in metal-on-vitamin-E-infused polyethylene bearings. Roentgen stereophotogrammetric analysis (RSA) examinations were obtained postoperatively at 3, 12, and 24 months. The primary outcome was proximal cup migration when comparing the 2 cup surfaces and also when comparing the largest possible head with the 32-mm head. The patients were followed for 2 years.

RESULTS

The median (and interquartile range) proximal cup migration was 0.15 mm (0.02 to 0.32 mm) for the PTC cup and 0.21 mm (0.11 to 0.34 mm) for the PPS cup. The largest possible head had a proximal cup migration of 0.15 mm (0.09 to 0.31 mm), and the 32-mm head had a proximal cup migration of 0.20 mm (0.04 to 0.35 mm). There were no significant differences between the cup surface (p = 0.378) or the head size (p = 0.693) groups.

CONCLUSIONS

Early cup fixation was not superior with the novel PTC cup; the use of the largest possible head (36 to 44 mm) did not compromise early cup fixation.

LEVEL OF EVIDENCE

Therapeutic Level I. See Instructions for Authors for a complete description of levels of evidence.

A method to assess primary stability of acetabular components in association with bone defects

The objectives of this study were to develop a simplified acetabular bone defect model based on a representative clinical case, derive four bone defect increments from the simplified defect to establish a step-wise testing procedure, and analyze the impact of bone defect and bone defect filling on primary stability of a press-fit cup in the smallest defined bone defect increment. The original bone defect was approximated with nine reaming procedures and by exclusion of specific procedures, four defect increments were derived. The smallest increment was used in an artificial acetabular test model to test primary stability of a press-fit cup in combination with bone graft substitute (BGS). A primary acetabular test model and a defect model without filling were used as reference. Load was applied in direction of level walking in sinusoidal waveform with an incrementally increasing maximum load (300 N/1000 cycles from 600 to 3000 N). Relative motions (inducible displacement, migration, and total motion) between cup and test model were assessed with an optical measurement system. Original and simplified bone defect volume showed a conformity of 99%. Maximum total motion in the primary setup at 600 N (45.7 ± 5.6 µm) was in a range comparable to tests in human donor specimens (36.0 ± 16.8 µm). Primary stability was reduced by the bone defect, but could mostly be reestablished by BGS-filling. The presented method could be used as platform to test and compare different treatment strategies for increasing bone defect severity in a standardized way.

How screw connections influence the primary stability of acetabular cups under consideration of different bone models

Introduction

Good osteointegration of implants requires sufficient primary stability. Aim of this study was to examine the influence of screw fixation on the primary stability of press-fit cups.

Methods

Two press-fit acetabular cups were tested with regard to the influence and number of screws and their primary stability.

Results

For the relatively thin-walled Allofit®-S cup, an influence of the number of screws on the different forms of movement could be demonstrated.

Conclusions

We see a clear influence of the cup wall thickness on the elastic deformability and accordingly on the primary stability of the examined cups.

Clinical evaluation of a novel press-fit acetabular cup using "Ein-Bild-Roentgen-Analysis" (EBRA): A positive short-term prognosis

Introduction

Demographic change and demand for high quality of life lead to increasing implantation numbers. Aim of this study was to compare the Plasmafit® cup to Allofit® and Plasmacup®.

Methods

The study included 174 patients who had received 33 Plasmacup®, 68 Allofit® and 73 Plasmafit® cup implants. These were reviewed postoperatively, after 6 months control and after 12 months.

Results

No significant progressive migration could be discovered in any of the cup systems. At each follow-up the cups showed nearly constant values.

Conclusions

All examined acetabular cups showed excellent migration behavior within the first 12 postoperative months.

Results of 200 Consecutive Ceramic-on-Ceramic Cementless Hip Arthroplasties in Patients Up To 50 Years of Age: A 5-24 Years of Follow-Up Study

BACKGROUND

Ceramic-on-ceramic (CoC) bearings due to their mechanical properties can be used in primary total hip arthroplasty (THA) especially in young patients requiring a long-lasting implant. The goal of this study is to analyze the results of a series of 200 consecutive CoC THAs in patients aged 50 years or less at the time of surgery.

METHODS

A retrospective study was conducted on the first 200 consecutive CoC arthroplasties performed using the direct lateral approach on 105 females and 81 males (14 bilateral cases) with an average age of 44.2 (16-50) years. The diagnosis was primary or post-traumatic osteoarthritis in 94 cases, avascular necrosis of the femoral head in 47, displaced intracapsular femoral neck fracture in 29, osteoarthritis secondary to developmental dysplasia of the hip/Legg-Calvè-Perthes disease/slipped capital femoral epiphysis in 20, and rheumatic diseases in 10 cases. The preoperative Harris Hip Score was 32.5 on average (range 15-55). All the implants were cementless. In 177 THAs the coupling was alumina-on-alumina, and in 23 cases the coupling was AMC-on-AMC (alumina matrix composite).

RESULTS

Twenty-five patients with 28 THAs were lost at the final follow-up, 2 stems were revised due to subsidence, 1 cup was revised due to malposition, 1 femoral head was changed because of impingement, and 1 THA was revised for periprosthetic infection. Three patients sustained a Vancouver B1 periprosthetic femoral fracture. At the final follow-up (mean 14.9 years; minimum 5 years to maximum 24 years), 172 THAs were eligible for clinical and radiographic evaluation: none was revised for wear and/or breakage of the ceramic components. Harris Hip Score rose up to a mean value of 90.1 (52-100).

CONCLUSION

The present report demonstrates that CoC coupling offers excellent long-term results in THA performed in young patients with very low wear and no adverse effects caused by the material.