Better primary stability with porous titanium particles than with bone particles in cemented impaction grafting: an in vitro study in synthetic acetabula

Primary stability of a press-fit cup in combination with impaction grafting in an acetabular defect model

The objectives of this study were to (a) assess primary stability of a press-fit cup in a simplified acetabular defect model, filled with compacted cancellous bone chips, and (b) to compare the results with primary stability of a press-fit cup combined with two different types of bone graft substitute in the same defect model. A previously developed acetabular test model made of polyurethane foam was used, in which a mainly medial contained defect was implemented. Three test groups (N = 6 each) were prepared: Cancellous bone chips (bone chips), tricalciumphosphate tetrapods + collagen matrix (tetrapods + coll), bioactive glass S53P4 + polyethylene glycol-glycerol matrix (b.a.glass + PEG). Each material was compacted into the acetabulum and a press-fit cup was implanted. The specimens were loaded dynamically in the direction of the maximum resultant force during level walking. Relative motion between cup and test model was assessed with an optical measurement system. At the last load step (3000 N), inducible displacement was highest for bone chips with median [25th percentile; 75th percentile] value of 113 [110; 114] µm and lowest for b.a.glass + PEG with 91 [89; 93] µm. Migration at this load step was highest for b.a.glass + PEG with 868 [845; 936] µm and lowest for tetrapods + coll with 491 [487; 497] µm. The results show a comparable behavior under load of tetrapods + coll and bone chips and suggest that tetrapods + coll could be an attractive alternative to bone chips. However, so far, this was found for one specific defect type and primary stability should be further investigated in additional/more severe defects.

Primary Stability of Revision Acetabular Reconstructions Using an Innovative Bone Graft Substitute: A Comparative Biomechanical Study on Cadaveric Pelvises

Hip implant failure is mainly due to aseptic loosening of the cotyle and is typically accompanied by defects in the acetabular region. Revision surgery aims to repair such defects before implantation by means of reconstruction materials, whose morselized bone graft represents the gold standard. Due to the limited availability of bone tissue, synthetic substitutes are also used. The aim of this study was to evaluate if a synthetic fully resorbable tri-calcium phosphate-based substitute can provide adequate mechanical stability when employed to restore severe, contained defects, in comparison with morselized bone graft. Five cadaveric pelvises were adopted, one side was reconstructed with morselized bone graft and the other with the synthetic substitute, consisting of dense calcium phosphate granules within a collagen matrix. During the biomechanical test, cyclic load packages of increasing magnitude were applied to each specimen until failure. Bone/implant motions were measured through Digital Image Correlation and were expressed in terms of permanent and inducible translations and rotations. The reconstruction types exhibited a similar behavior, consisting of an initial settling trend followed by failure as bone fracture (i.e., no failure of the reconstruction material). When 2.2 Body Weight was applied, the permanent translations were not significantly different between the two reconstructions (p = 0.06–1.0) and were below 1.0 mm. Similarly, the inducible translations did not differ significantly (p = 0.06–1.0) and were below 0.160 mm. Rotations presented the same order of magnitude but were qualitatively different. Overall, the synthetic substitute provided adequate mechanical stability in comparison with morselized bone graft, thus representing a reliable alternative to treat severe, contained acetabular defects.

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.

The biological approach in acetabular revision surgery: impaction bone grafting and a cemented cup

Acetabular impaction bone grafting (IBG) in combination with a cemented cup in revision total hip arthroplasty (THA) is a proven and well-recognised technique which has been used in clinical practice for more than 35 years. Nowadays, with cemented prostheses tending to lose a larger part of the THA market every year in primary and revision cases, and many young surgeons being only trained in implanting uncemented prostheses, this technique is considered by many as technically demanding and time consuming, making its use less appealing. Despite this image and many new innovative techniques using uncemented implants in acetabular revisions over the last 25 years, IBG with a cemented cup is still one of the few techniques that really can reconstitute bone and respects human biology. In this era of many biologically-based breakthroughs in medicine, it is hard to explain that the solution of most orthopaedic surgeons for the extensive bone defects as frequently seen during acetabular revision surgery, consists of implanting bigger and larger metal implants. This review aims to put the IBG method into a historical perspective, to describe the surgical technique and present the clinical results.

Porous titanium particles for acetabular reconstruction in total hip replacement show extensive bony armoring after 15 weeks. A loaded in vivo study in 10 goats

BACKGROUND AND PURPOSE

The bone impaction grafting technique restores bone defects in total hip replacement. Porous titanium particles (TiPs) are deformable, like bone particles, and offer better primary stability. We addressed the following questions in this animal study: are impacted TiPs osteoconductive under loaded conditions; do released micro-particles accelerate wear; and are systemic titanium blood levels elevated after implantation of TiPs?

ANIMALS AND METHODS

An AAOS type-III defect was created in the right acetabulum of 10 goats weighing 63 (SD 6) kg, and reconstructed with calcium phosphate-coated TiPs and a cemented polyethylene cup. A stem with a cobalt chrome head was cemented in the femur. The goats were killed after 15 weeks. Blood samples were taken pre- and postoperatively.

RESULTS

The TiP-graft layer measured 5.6 (SD 0.8) mm with a mean bone ingrowth distance of 2.8 (SD 0.8) mm. Cement penetrated 0.9 (0.3-1.9) mm into the TiPs. 1 reconstruction showed minimal cement penetration (0.3 mm) and failed at the cement-TiP interface. There were no signs of accelerated wear, metallic particle debris, or osteolysis. Median systemic titanium concentrations increased on a log-linear scale from 0.5 (0.3-1.1) parts per billion (ppb) to 0.9 (0.5-2.8) ppb (p=0.01).

INTERPRETATION

Adequate cement pressurization is advocated for impaction grafting with TiPs. After implantation, calcium phosphate-coated TiPs were osteoconductive under loaded conditions and caused an increase in systemic titanium concentrations. However, absolute levels remained low. There were no signs of accelerated wear. A clinical pilot study should be performed to prove that application in humans is safe in the long term.