Bone ongrowth and mechanical fixation of implants in cortical and cancellous bone.

BACKGROUND: What is the right surface for an implant to achieve biological fixation? Surface technologies can play important roles in encouraging interactions between the implant surface and the host bone to achieve osseointegration. Preclinical animal models provide important insight into in vivo performance related to bone ongrowth and implant fixation. METHODS: A large animal model was used to compare the in vivo response of HA and plasma-sprayed titanium coatings in a well-reported adult ovine model to evaluate bone ongrowth in terms of mechanical properties in cortical sites, and histology and histomorphometry in cortical and cancellous sites at 4 and 12 weeks. RESULTS: Titanium plasma-sprayed surfaces outperformed the HA-coated samples in push-out testing in cortical sites while both surfaces supported new bone ongrowth and remodeling in cortical and cancellous sites. CONCLUSIONS: While both HA and Ti plasma provided an osteoconductive surface for bone ongrowth, the Ti plasma provided a more robust bone-implant interface that ideally would be required for load transfer and implant stability in the longer term.

The in vivo response to a novel Ti coating compared with polyether ether ketone: evaluation of the periphery and inner surfaces of an implant.

BACKGROUND CONTEXT: Increasing bone ongrowth and ingrowth of polyether ether ketone (PEEK) interbody fusion devices has the potential to improve clinical outcomes. PURPOSE: This study evaluated the in vivo response of promoting new bone growth and bone apposition with NanoMetalene (NM) compared with PEEK alone in a cancellous implantation site with an empty aperture. STUDY DESIGN: This is a randomized control animal study. METHODS: Implants and funding for this study were provided by SeaSpine (60,000 USD). Cylindrical dowels with two apertures were prepared as PEEK with a sub-micron layer of the titanium (NM). The titanium coating was applied over the entire implant (Group 1) or just the apertures (Group 2). Polyether ether ketone implants with no coating served as controls (Group 3). Implants were placed in the cancellous bone of the distal femur or proximal tibia with no graft material placed in the apertures in eight adult sheep. Bone ongrowth to the surface of the implant and ingrowth into the apertures was assessed at 4 and 8 weeks after surgery with micro-computed tomography (CT) and undecalcified histology. RESULTS: The apertures in the implants were notably empty in the PEEK group at 4 and 8 weeks. In contrast, new bone formation into the apertures was found in samples coated with NM even though no graft material was placed into the defect. The bone growing into the aperture tracked along the titanium layer. Apertures with the titanium coating demonstrated significantly more bone by micro-CT qualitative grading compared with PEEK with average bone coverage scores of Group 1 (NM) 1.62±0.89, Group 2 (NM apertures only) 1.62±0.77, and Group 3 (PEEK) 0.43±0.51, respectively, at 4 weeks (p<.01) and Group 1 (NM) 1.79±1.19, Group 2 (NM apertures only) 1.98±1.18, and Group 3 (PEEK) 0.69±0.87, respectively, at 8 weeks (p<.05). The amount of bone in the apertures (ingrowth) quantified using the volumetric data from the micro-CT supported an overall increase in bone volume inside the apertures with the titanium coating compared with PEEK. Histology showed newly formed woven bone tracked along the surface of the titanium in the apertures. The PEEK interface presented the typical nonreactive fibrous tissue inside the apertures at 4 weeks and some focal contact with bone on the outside at 4 weeks and 8 weeks. CONCLUSIONS: Micro-CT and histology demonstrated bone ongrowth to the surfaces coated with NM where the newly formed bone tracked along the thin titanium-coated surfaces. Polyether ether ketone surfaces presented the nonreactive fibrous tissue at the interface as previously reported in preclinical scenarios.

Animal Models for Tendon Repair Experiments: A Comparison of Pig, Sheep and Human Deep Flexor Tendons in Zone II.

BACKGROUND: This laboratory study compared pig, sheep and human deep flexor tendons in regards to their biomechanical comparability. METHODS: To investigate the relevant biomechanical properties for tendon repair experiments, the tendons resistance to cheese-wiring (suture drag/splitting) was assessed. Cheese-wiring of a suture through a tendon is an essential factor for repair gapping and failure in a tendon repair. RESULTS: Biomechanical testing showed that forces required to pulling a uniform suture loop through sheep or pig tendons in Zone II were higher than in human tendons. At time point zero of testing these differences did not reach statistical significance, but differences became more pronounced when forces were measured beyond initial cheese-wiring (2 mm, 5 mm and 10 mm). The stronger resistance to cheese-wiring was more pronounced in the pig tendons. Also regarding size and histology, sheep tendons were more comparable to human tendons than pig tendons. CONCLUSIONS: Differences in tendon bio-properties should be kept in mind when comparing and interpreting the results of laboratory tendon experiments.

The contribution of the cortical shell to pedicle screw fixation.

BACKGROUND: A pedicle screw insertion technique known as "hubbing" involves the removal of cortical bone around the screw insertion with the aim of improving fixation and decreasing screw loosening. However, the efficacy of this procedure relative to bone density and early loading have not been fully explored. The purpose of this study is to establish the contribution of the cortical layer (hubbing), cancellous density, early loading (toggling) in an idealised model. This is an in vitro laboratory study. METHODS: Synthetic bone blocks with cancellous bulk and a simulated cortical shell were implanted with 6.5 mm pedicle screws. Three key variables were evaluated in this study; density of the simulated bone (10-20 lb/ft3), toggling (±0.5 mm for 10,000 cycles), and the presence or absence of the surrounding cortex (hubbing). Pullout testing after toggling was performed to determine maximum load, stiffness and energy. Results were analyzed to assess interaction and main effects. RESULTS: Removal of the cortex decreased the pullout loads by approximately 1,100 N after toggling. Toggling in the presence of the cortical shell had no effect. However, once the cortical shell is removed damage to the weaker cancellous bone accumulates and further compromises the fixation. CONCLUSIONS: The addition of a cortical layer in the Sawbone model is significant and provides a more realistic model of load sharing. The cortex plays a considerable role in the protection of underlying cancellous bone as well as contributing to initial pullout strength. The results of this study demonstrate a negative synergistic effect when both toggling and hubbing are applied to the weaker bone.

Critical Size Bone Defect Healing Using Collagen-Calcium Phosphate Bone Graft Materials.

The need for bone graft materials to fill bony voids or gaps that are not related to the intrinsic stability of the bone that arise due to trauma, tumors or osteolysis remains a clinically relevant and significant issue. The in vivo response of collagen-tricalcium phosphate bone graft substitutes was evaluated in a critical size cancellous defect model in skeletally mature rabbits. While the materials were chemically virtually identical, new bone formation, implant resorption and local in vivo responses were significantly different. Differences in the in vivo response may be due, in part, collagen source and processing which influences resorption profiles. Continued improvements in processing and manufacturing techniques of collagen-tricalcium phosphate bone graft substitutes can result in osteoconductive materials that support healing of critical size bone defects even in challenging pre-clinical models.

Structural Failure Mechanisms of Common Flexor Tendon Repairs.

BACKGROUND: This study investigated the exact failure mechanisms of the most commonly used conventional tendon repair techniques. A new method, radiographing repair constructs in antero-posterior and lateral projections before and after tensioning was used. This allowed to precisely analyse failure mechanisms in regards to geometrical changes in all three dimensions. Additionally the biomechanical stability focusing on gapping was tested. METHODS: Sheep fore limb deep flexor tendons were harvested and divided in eight groups of ten tendons. Three common variants of the Kessler repair method and four common 4-strand repair techniques were tested. Additionally a new modification of the Adelaide repair was tested. RESULTS: Biomechanical testing showed no significant differences in gapping for the three tested 2-strand Kessler repair groups. Once a double Kessler or 4-strand Kessler repair was performed the stability of the repair improved significantly. Further significant improvements in biomechanical stability could be achieved by using cross locks in the repair like in the Adelaide repair method. Qualitative analysis using radiographs showed that all Kessler repair variants unfolded via rotations around the transverse suturing component, no matter which variant was used. CONCLUSIONS: Additional to the commonly described constriction of the repair construct, the rotating deformation is the main reason for repair site gapping in Kessler tendon repair methods. The term "locking" in a Kessler repair is misleading. The cruciate repairs tended to loose grip and drag (cheese-wire) through the tendon and therefore lead to gapping. The most stable repair constructs in all three dimensions were the Adelaide repair and its interlocking modification. This is due to the superior anchoring qualities of its cross locks and three dimensional stability.

Variations in the trunnion surface topography between different commercially available hip replacement stems.

Modular hip implants allows for the adjustment of leg length, offset, and the ability to remove the head for acetabular exposure during primary and revision surgery. The design of the Morse taper facilitates the intimate contact of the conical trunnion of the femoral stem (male component), with the conical bore of the femoral head (female component). Orthopaedic trunnion tapers are not standardized and vary in length, taper angle, and base dimension. Variations in the design and surface characteristics of the trunnion, will directly reflect on the interface at the taper junction and can influence the likelihood of subsequent wear, corrosion and longevity of the implant. The effect of surface topography of trunnions on commercially available hip stems has not yet been considered as a possible contributing factor in the corrosion observed at taper junctions. In this study we analyzed the surface topography and surface roughness of randomly selected commercially available femoral hip stem trunnions to obtain a greater insight into their surface characteristics.

A sheep model for cancellous bone healing.

Appropriate well-characterized bone defect animal models remain essential for preclinical research. This pilot study demonstrates a relevant animal model for cancellous bone defect healing. Three different defect diameters (8, 11, 14 mm) of fixed depth (25 mm) were compared in both skeletally immature (18-month-old) and aged sheep (5-year-old). In each animal, four defects were surgically created and placed in the cancellous bone of the medial distal femoral and proximal tibial epiphyses bilaterally. Animals were euthanized at 4 weeks post-operatively to assess early healing and any biological response. Defect sites were graded radiographically, and new bone formation quantified using µCT and histomorphometry. Fibrous tissue was found within the central region in most of the defects with woven bone normally forming near the periphery of the defect. Bone volume fraction [bone volume (BV)/TV] significantly decreased with an increasing defect diameter. Actual BV, however, increased with defect diameter. Bone ingrowth was lower for all defect diameters in the aged group. This pilot study proposes that the surgical creation of 11 mm diameter defects in the proximal tibial and distal femoral epiphyses of aged sheep is a suitable large animal model to study early healing of cancellous bone defects. The refined model allows for the placement of four separate bone defects per animal and encourages a reduction in animal numbers required for preclinical research.

Wound infusion with local anaesthesia after laparotomy: a randomized controlled trial.

BACKGROUND: The use of a continuous local anaesthesia infusion after laparotomy may reduce opioid requirements and facilitate earlier return of bowel function, independent mobilization and hospital discharge. METHODS: We performed a double-blinded, randomized controlled trial on 55 patients who underwent laparotomy. Patients were randomly allocated to receive a continuous infusion of either 0.2% ropivacaine or normal saline into their midline abdominal wound at the fascial level. The end points of the study were: total opioid requirements at 24 and 48 h; time to first flatus, bowel movement and independent ambulation; length of hospital stay; complications; and daily mean patient-reported pain scores at rest and movement. RESULTS: The two treatment groups were well controlled for factors that influence analgesia requirements, including age, weight, length of wound incision and type of operation. Patients allocated to ropivacaine infusion used, on average, 32 mg less morphine at 48 h (95% confidence interval 7, 57; P= 0.01). This was highly statistically significant after adjusting for age, gender and type of operation (P= 0.0006). Ropivacaine infusion was associated with a significantly decreased time to independent mobilization (P= 0.02), time to first flatus (P= 0.02) and reduced post-operative ileus (2/28 versus 9/27, χ(2) = 5.89, P= 0.02). There was no significant effect of ropivacaine infusion on time to first bowel movement (P= 0.94) nor length of hospital stay (P= 0.77). CONCLUSIONS: Local anaesthesia infusion at the fascial plane provides effective analgesia. This improves patient recovery through earlier return to bowel function and mobilization.

Posterolateral spinal fusion in a rabbit model using a collagen-mineral composite bone graft substitute.

Choosing the appropriate graft material to participate in the healing process in posterolateral spinal fusion continues to be a challenge. Combining synthetic graft materials with bone marrow aspirate (BMA) and autograft is a reasonable treatment option for surgeons to potentially reduce or replace the need for autograft. FormaGraft, a bone graft material comprising 12% bovine-derived collagen and 88% ceramic in the form of hydroxyapatite (HAp) and beta tricalcium phosphate (beta-TCP) was evaluated in three possible treatment modalities for posterior spinal fusion in a standard rabbit model. These three treatment groups were FormaGraft alone, FormaGraft soaked in autogenous BMA, and FormaGraft with BMA and iliac crest autograft. No statistically demonstrable benefits or adverse effects of the addition of BMA were found in the current study based on macroscopic, radiology or mechanical data. This may reflect, in part, the good to excellent results of the collagen HA/TCP composite material alone in a well healing bony bed. Histology did, however, reveal a benefit with the use of BMA. Combining FormaGraft with autograft and BMA achieved results equivalent to autograft alone. The mineral and organic nature of the material provided a material that facilitated fusion between the transverse processes in a standard preclinical posterolateral fusion model.

Mechanical properties of reconstructed achilles tendon with transfer of peroneus brevis or flexor hallucis longus tendon.

Treatment of chronic Achilles tendon ruptures can be technically difficult because of tendon retraction, atrophy, and short distal stumps. Surgical repair of chronic Achilles tendon ruptures focuses on local and free tendon transfers, as well as reconstruction with allografts or synthetic materials. This study examined the in vitro mechanical properties of a reconstructed Achilles tendon with the peroneus brevis or the flexor hallucis longus tendons in a human cadaver model. The tendons were harvested from 17 fresh-frozen human cadavers, and the same techniques were used for all of the model reconstructions. Biomechanical measurements included the failure load, stiffness, energy-to-peak load, and mode of failure. The mean failure load was significantly higher in the peroneus brevis group (P = .036), and there was no significant difference in stiffness and energy-to-peak load between the peroneus brevis and flexor hallucis longus groups. In every case, the mode of failure involved the tendon graft pulling through either the distal or proximal stump of the Achilles tendon. The greater failure loads observed with the use of peroneus brevis may not be clinically relevant, however, because of the magnitude of the peak loads observed in the cadaveric model. The present study supports the use of either peroneus brevis or flexor hallucis longus for reconstruction of chronic Achilles tendon ruptures and indicates the need for surgeons to carefully reinforce the attachment of the transferred tendon grafts to the stumps of the Achilles tendon to prevent pullout.

The immunolocalisation of VEGF in the articular cartilage of sheep mandibular condyles.

INTRODUCTION: Vascular endothelial growth factor (VEGF) has recently been found to be essential for hypertrophic chondrocyte apoptosis and angiogenesis at the growth plate of long bones, indicating a central role in endochondral ossification. VEGF has more recently, also been shown to be expressed in articular cartilage chondrocytes in human osteoarthritic and rheumatoarthritic joints but not healthy adult joints. To investigate the role of VEGF in the fibrocartilage of the temporomandibular joint, this study aimed to document the presence and distribution of VEGF in the condylar articular cartilage of sheep temporomandibular joints. METHODS: Mandibular condyles of the temporomandibular joints of five 18-month old Wether sheep were fixed, decalcified, paraffin embedded and sectioned. The sections were analyzed using immunohistochemistry for VEGF. RESULTS: VEGF was found to be localised predominantly to the proliferative and maturing layers of chondrocytes in the condylar fibrocartilage of the temporomandibular joints. Articular cartilage is an avascular and alymphatic tissue. As such, the localisation of VEGF to the articular cartilage of normal temporomandibular joint condyles suggests a role for VEGF other than angiogenesis. CONCLUSION: VEGF is shown here for the first time to be present in mandibular condylar cartilage, leading us to propose a possible role in non-angiogenic extracellular matrix remodeling.