Synergetic effect of freeze-drying and gamma irradiation on the mechanical properties of human cancellous bone.

Freeze-drying and irradiation are common process used by tissue banks to preserve and sterilize bone allografts. Freeze dried irradiated bone is known to be more brittle. Whether bone brittleness is due to irradiation alone, temperature during irradiation or to a synergetic effect of the freeze-drying-irradiation process was not yet assessed. Using a left-right femoral head symmetry model, 822 compression tests were performed to assess the influence of sequences of a 25 kGy irradiation with and without freeze-drying compared to the unprocessed counterpart. Irradiation of frozen bone did not cause any significant reduction in ultimate strength, stiffness and work to failure. The addition of the freeze-drying process before or after irradiation resulted in a mean drop of 35 and 31% in ultimate strength, 14 and 37% in stiffness and 46 and 37% in work to failure. Unlike irradiation at room temperature, irradiation under dry ice of solvent-detergent treated bone seemed to have no detrimental effect on mechanical properties of cancellous bone. Freeze-drying bone without irradiation had no influence on mechanical parameters, but the addition of irradiation to the freeze-drying step or the reverse sequence showed a detrimental effect and supports the idea of a negative synergetic effect of both procedures. These findings may have important implications for bone banking.

Particle size influence in an impaction bone grafting model. Comparison of fresh-frozen and freeze-dried allografts.

BACKGROUND: Impaction bone grafting with large particles is considered as mechanically superior to smaller morsels. Interest of freeze-dried irradiated bone for impaction bone grafting has been observed with small particles. Influence of bone process on other particle sizes still needed to be assessed. MATERIAL AND METHODS: Twenty-four osteoarthrotic femoral heads were used to prepare fresh-frozen and freeze-dried irradiated cancellous bone. Each group was divided into four batches of different particle sizes, each batch containing 18 samples. The different particle sizes were obtained with a Retsch Cross Beater Mill SK 100, Noviomagus rotating bone mills with two sizes of rasps and a Luer bone rongeur. Bone grafts were impacted in a contained cylinder. Stiffness was monitored during impaction. RESULTS: Freeze-dried irradiated grafts showed higher stiffness than fresh-frozen bone whatever the size of the particles. Large particles obtained with the rongeur and the large rasp from the Noviomagus bone mill were mechanically superior than small particles up to 30 impactions. INTERPRETATION: Large particles offer better mechanical performance as a greater magnitude of force would be required to deform and break the particles. Freeze-dried irradiated bone brittleness reduces this advantage after 30 impactions. Large particles embrittlement leads to similar mechanical results as small particles at higher impaction rate. This may account for partial collapse of the graft layer in clinical situation when impaction rate is lower. This model supports the use of small particles obtained with thin rasp bone mill when freeze-dried irradiated bone for impaction bone grafting and large particles obtained with the Rongeur when fresh-frozen bone is available.

Neck fracture femoral heads for impaction bone grafting: evolution of stiffness and compactness during impaction of osteoarthrotic and neck-fracture femoral heads.

BACKGROUND: The need for safe bone allografts is increasing and preservation of femoral heads from patients being operated on with hip arthroplasty should be encouraged. However, should we preserve femoral heads from patients operated on for neck fracture as tissue mechanical quality may not be satisfactory? MATERIAL AND METHODS: We compared the evolution of stiffness and compactness of fresh-frozen morselized bone obtained from osteoarthrotic femoral heads and those from neck fractures. Both materials were also compared after freeze-drying and irradiation. We used 6 osteoarthrotic and 6 neck-fracture femoral heads to prepare 4 batches of morselized bone. 18 samples from each batch were impacted in a contained cylinder. Frozen bone grafts were tested after thawing at room temperature for 2 hours and freeze-dried grafts were tested after 30 minutes of rehydration. RESULTS: The stiffness of fresh-frozen neck fracture bone was lower than that of fresh-frozen osteoarthrotic bone at 150 impactions. The stiffness of freeze-dried irradiated bone was higher than that of the fresh-frozen bone and did not differ between osteoarthrotic and neck-fracture bone. INTERPRETATION: Solvent-treated freeze-dried bone from femoral heads procured during arthroplasty for sub-capital hip fractures represents a valuable source of material for allografts, addressing concerns regarding serological testing, medical history and bone quality.

Freeze-dried irradiated bone brittleness improves compactness in an impaction bone grafting model.

BACKGROUND: Defatted bone chips with or without freeze-drying and irradiation have mechanical advantages as compared to fresh-frozen controls in in vitro models of impaction. These improved results have been ascribed to replacement of viscous bone marrow by saline and embrittlement of the freeze-dried bone by irradiation. MATERIAL AND METHODS: To determine which of these hypotheses is correct, we compared the development of stiffness and compactness of morselized bone graft that had been: 1) fat-reduced with saline, and 2) fresh-frozen, solvent-detergent defatted, 3) freeze-dried irradiated and 4) not irradiated. We used 12 osteoarthrotic femoral heads to prepare these four batches of morselized bone, and impacted 18 samples from each batch in a cylinder. The frozen bone grafts were tested after thawing at room temperature for 2 hours and the freeze-dried grafts were tested after 30 minutes of rehydration. We monitored the development of compactness and stiffness of the material during impaction. RESULTS: The stiffness of the freeze-dried irradiated bone was greater than that of the other three series after 10, 50 and 150 impactions. The freeze-dried bone chips that were not irradiated and the chips defatted with saline alone were less stiff than the fresh-frozen control after 150 impactions. INTERPRETATION: The brittleness of freeze-dried irradiated bone, caused by loss of the capacity to absorb energy in a plastic way, increases the compactness and stiffness of the morselized grafts. Washing bone with saline alone or treating bone with solvent-detergent but no irradiation had no similar mechanical advantage and the bone did not impact better than fresh-frozen undefatted bone in our model.

Impaction bone grafting with freeze-dried irradiated bone. Part II. Changes in stiffness and compactness of morselized grafts: experiments in cadavers.

In the technique of impaction bone grafting, implant stability depends on the mechanical properties of the impacted morselized grafts. Although the procedure is usually performed with fresh-frozen femoral heads, there is still some concern about their supply and safety. Bone processing is a potential solution, but the mechanical properties of this material during and after impaction need to be determined. We used 6 osteoarthrotic femoral heads to prepare two paired batches of morselized bone. One batch was morselized and frozen. The other batch was chemically treated, morselized, freeze-dried and then gamma-irradiated. We impacted 18 samples from each batch in a contained cylinder. Freeze-dried bone grafts were tested after 30 minutes of rehydration. The changes in the compactness and stiffness of the material were monitored during the impaction. The compaction of the freeze-dried bone was faster than that of their fresh-frozen control. The maximal stiffness reached by both materials was the same (55 MPa), but the freeze-dried grafts required three to four times fewer impactions to achieve that stiffness. After 3, 10 and 50 impactions the freeze-dried bone was stiffer than the fresh-frozen bone. As it is easier to impact, the freeze-dried bone may be mechanically more efficient than the fresh-frozen bone in surgical conditions. Moreover, the processed bone meets the highest safety standards, as regards the risk of disease transmission.

Impaction bone grafting with freeze-dried irradiated bone. Part I. Femoral implant stability: cadaver experiments in a hip simulator.

Processed freeze-dried irradiated allografts seem to be used less than instead of fresh-frozen allografts for impaction bone grafting in revision hip arthroplasties. Although biologically acceptable, their use is discouraged because of their questionable mechanical properties following freeze-drying and irradiation procedures. To address this question, we impacted freeze-dried grafts in 6 cadaveric femurs and loaded with a cemented Charnley prosthesis. The routinely used fresh-frozen allografts were used as controls in the contralateral side. These constructs were compared simultaneously in a walking hip simulator for their stability during 900,000 loading cycles. The mechanical parameters were axial inducible displacement and subsidence of the implant. The former parameter was lower in the implant mounted on freeze-dried impacted grafts than that mounted on the fresh-frozen bone. The latter parameter was also lower in the freeze-dried group. At the end of the test, we found no implant loosening in either group and their 'pull out' resulted in cement-prosthesis debonding, which showed the mechanical integrity of the impacted grafts. Freeze-dried grafts provide more stable fixation of the stem than fresh-frozen morselized grafts, when tested in a hip simulator.