Background

Processing of allografts, which are used to fill bone defects in orthopaedic surgery, includes chemical cleaning as well as gamma irradiation to reduce the risk of infection. Viable bone cells are destroyed and denaturing proteins present in the graft the osteoconductive and osteoinductive characteristics of allografts are altered. The aim of the study was to investigate the mechanical differences of chemical cleaned allografts by adding blood, clotted blood, platelet concentrate and platelet gel using a uniaxial compression test.

Introduction

BAG-S53P4 has similar mechanical properties as cortical bone tissue and can be used as an additive to bone allografts. The aim of this study was to evaluate the effect of adding BAG-S53P4 to chemically treated allografts with controlled grain size distribution.

A cleaning process reduces the contamination risk in bone impaction grafting but also modifies the grain size distribution. The cleaned allograft shows a higher mechanical stability than the untreated group.

In revision total hip replacement, bone loss can be managed by impacting porous bone chips. The bone chips have to be compacted to guarantee sufficient mechanical strength. To improve the safety of bone grafts and to reduce the risk of bacterial and viral contamination, cleaning processes are used to remove the organic portion of the tissue while maintaining its mechanical characteristics.

A cleaning procedure described by Coraca-Huber et al. was compared to untreated allografts by performing a sieve analysis, followed by an uniaxial compression test. Differences in grain size distribution and weight loss during the cleaning procedure were compared to data from literature. Yield stress limits, flowability coefficients as well as initial density and density at the yield limit of the two groups were determined for each group over 30 measurements. The measurements were taken before and after compression with an impaction apparatus (dropped weight).

The cleaning process reduced the initial weight by 56%, which is comparable to the results of McKenna et. al. Cleaned allograft showed a 25% lower weight of bone chips sized > 4 mm compared to data from a previous study.

The cleaned bone chips showed a statistically significant (p > 0.01) higher yield limit to a compression force (0.165 ± 0.069 MPa) compared to untreated allograft after compaction (0.117 ± 0.062 MPa). The flowability coefficient was 0.024 for the cleaned allograft and 0.034 for the untreated allograft.

Initial density as well as the density at the yield limit was higher for the untreated allografts, as the sample weight was twice as high as in the cleaned group, to compensate for the washout of the organic portion. The cleaned bone grafts showed a higher compaction rate, which was 31%, compared the the untreated group with a compaction rate of 22%.

The cleaned allograft showed a higher compaction rate, which means that the gaps between the single grains are filled out with smaller particles, resulting in better interlocking. In the untreated allograft the interlocking mechanism is hindered by the organic elements. This observation is confirmed by a reduced flowabillity and a higher yield stress limit. The loss of weight as well as a higher compaction rate implies that more cleaned graft material is needed to fill bone defects in hip surgery. Sonication may damage the bone structure of the allograft and reduce the size of the particles.

In minimally invasive direct anterior total hip arthroplasty double offset broach handles are used, in order to facilitate the preparation of the femoral canal. The maximum value of the main force peak and the impulse of two types of double offset broach handles (A European version, B American version) were compared to a single offset broach handle (S). Results have demonstrated that the highest values of the main force peak and force impulse were found in the single offset broach handle. Broach handle A had higher impulse values and lower maximum force values compared to broach handle B. In double offset broach handles less energy is transmitted to the tip. Broach handle A has a lower force peak than B and therefore a reduced risk of bone fracture.

In Total Hip Arthroplasty (THA) bone loss is recovered by using compacted porous bone chips. The technique requires the morsellised allograft to be adequately compacted to provide initial stability for the prosthesis in order to prevent early massive subsidence and to induce bone remodeling. Therefore the bone grafts provide initial stability and an environment in which revascularization and incorporation of the graft into the host skeleton may occur. Acetabular reconstruction with impacted morsellised cancellous grafts and cement leads to satisfactory long-term results. In the acetabular impact-grafting procedure, a hammer and an impaction stick is used for manual compaction. Another technique uses a hammer driven by compressed air, which could lead to higher density and improved stability of bone chips in the acetabulum. The aim of this study was to compare two different compaction modes for bone impaction grafting for the acetabulum. The hypothesis was that a pneumatic impaction method would produce less variable results than the manual impaction mode and lead to better compaction results of the bone chips in less time.

Bone mass characteristics were measured by force and distance variation of a penetrating punch, which was lowered into a plastic cup filled with bone chips. For each compaction method and for each time interval (0, 3, 6, 9, 12, 15 and 30 [s] of compaction time) 30 measurements of force and distance variations were taken. From the measurements of force and distance variations bulk density, contact stiffness, impaction hardness and penetration resistance were calculated before and after the established time intervals of compaction. Since not all data was normally distributed the non-parametric U-Test was used for comparison of the two impaction methods. Particle size distribution was determined using sieve analysis according to Din 18123 standard after the compaction experiments.

Results have shown that the pneumatic method leads to higher values in impaction hardness, contact stiffness and bulk density and is more suitable to increase the primary stability of the implant. The differences in bulk density, impaction hardness and contact stiffness where statistically significant (p<0.01). No significant differences were found between the two different methods concerning the penetration resistance. The coefficient of uniformity C_u, calculated from the particle size distribution determined by the sieve analysis, has a value of 3.8.

The particle size distribution is comparable to the results published in literature. Pneumatic impaction achieves higher density values in less time with less force applied and results in more reproducible outcomes when used. It reduces therefore the risk of bone fracture, as smaller peak forces are used for less time. However for optimal osteointegration it is not recommended to achieve maximum density. Further clinical studies should determine a reference value for optimal growth-in of osteocytes. Manual impaction shows more variable results and depends much on the experience of the surgeon. The pneumatic hammer is therefore a suitable tool to standardize the impaction process for acetabular bone defects.