Introduction

Hip and knee arthroplasty present surgeons with difficult bone loss. In these cases the use of morselized allograft is a well established way of optimizing early implant fixation. In revisions, the surgical field is potentially infected. The use of allograft bone creates a “dead space” in which the immune system has impaired access, and even a small amount of bacteria may therefore theoretically increase the risk of infection.

In vivo studies have shown that allograft bone is suitable as a vehicle of local antibiotic delivery.

We hypothesized that the allograft bone could be used as a local antibiotic delivery vehicle without impairing the implant fixation, tested by mechanical push-out.

Introduction: Hip arthroplasty can present surgeons with difficult bone loss. Impacted allografting is a well-established way of initally securing implant stability. However subsequent bone integration and fusion can be prolonged. Also concerns relate on maintaining bone volume of allograft during integration.

Intermittent administration of parathyroid hormone (PTH) is bone anabolic and improves fracture healing. As adjuvant in implant surgery PTH has only recently been introduced experimentally predominantly showing improved implant integration within empty peri-implant bone defects.

Given the desire to improve the graft incorporation process, the purpose of our study is to examine whether PTH improves early implant integration by accelerating healing of peri-implant bone allograft. We test the hypothesis that systemic intermittent administration of PTH increases new bone formation in allograft inserted in a gap with impacted morselized bone allograft around an experimental orthopaedic implant. We hypothesize that parathyroid hormone will improve new bone formation in allograft and preserve allograft.

Methods: An unpaired canine study was carried out following approval of our Institutional Animal Care and Use Committee. In 20 skeletally mature dogs cylindrical titanium alloy porous coated implants (6x10mm) were inserted in a 2.5 mm circumferential gap in the extraarticular cancellous bone site of the proximal humeri. Cancellous bone was milled on fine setting and impacted in the gap. Test animal were postoperatively randomised to daily treatment of placebo or parathyroid hormon rhPTH (1–34)(teriparatide)(Bachem) 5 μg / kg s.c. After 4 weeks observation time specimen blocks were harvested, sectioned and evaluated by unbiased stereological histomor-phometry (newCast, Visiopharm, Horsholm, Denmark). The endpoints were bone-to-implant contact and tissue density in an outer gap region of 1500 μm and an inner gap region reaching the implant. Since data were not normally distributed a non-parametric analysis two-sample Wilcoxon rank-sum test was applied with p-value < 0.05 considered statistically significant. Data are accordingly presented as median and interquartile ranges.

Results: Two implants in the PTH group were excluded. In the peri-centric region new bone improved significantly (outer region: PTH 21.1 (12.9–16.3) / control 15.2 (13.9–16.2), inner region: PTH 19.8 (15.8–21.5)/control 14.0 (12.9–16.3)). There were no significant differences in the amount of allograft. At the implant interface new bone for PTH was 11.5 (8.1–14.0), as for control 10.5 (7.2–14.8). Old bone for PTH was 1.5 (0.8–2.0), and old bone 1.4 (0.8–1.7). Bone tissue showed no significant differences.

Conclusion: Parathyroid hormone shows promise in significant inducing bone formation in impacted morselized allograft around implant without resorbing it significantly retaining graft volume.

Introduction: Treatment of osteoarthritis by total joint replacement generally shows a high success rate; however challenges remain. Prostheses inserted without cement are popular worldwide. Insertion of uncemented implants is intended to be pressfit. Early bone growth on the implant is critical to long-term fixation.

Parathyroid hormone (PTH) is a regulator of bone metabolism. When PTH is administered intermittently it induces strong anabolic effect by increasing osteoblastic activity. Our understanding of PTH is mainly based on research on osteoporosis, in which bone formation is known to be coupled to the bone resorption. In the orthopaedic situation of a joint replacement other conditions apply.

We therefore find it of interest to examine PTH’s role as an adjuvant in implant surgery. We examine the effect of PTH on the osseointegration of an experimental orthopaedic implant in which the implant due to insertion initiates a bone repair in the implant bed. We hypothesize that parathyroid hormone will improve the bone ongrowth at the bone-implant interface.

Methods: An unpaired canine study was carried out following approval of our Institutional Animal Care and Use Committee. In 20 skeletally mature dogs cylindrical titanium alloy porous coated implants (6×10mm) were inserted pressfit (0.1 mm under-drill) in the extraarticular cancellous bone site of the proximal tibia. Test animal were postoperatively randomised to daily treatment of placebo or parathyroid hormon rhPTH (1–34)(t eriparatide)(Bachem) 5 μg/kg s.c. After 4 weeks observation time specimen blocks were harvested, sectioned and evaluated by unbiased stereological histomorphometry (CAST-grid system (Olympus Denmark)). The endpoints were bone-to-implant contact and tissue density in a 500 μm region of interest. Since data were not normally distributed a non-parametric analysis two-sample Wilcoxon rank-sum test was applied with p-value < 0.05 considered statistically significant. Data are accordingly presented as median and interquartile ranges.

Results: Two implants in the PTH group were excluded. At the implant interface tissue density for PTH was 0,193 (0,157–0,229) for bone, 0,796 (0,764–0,821) for marrow and 0 (0–0,009) for fibrous tissue, as for control 0,163 (0,141–0,193) for bone, 0,837 (0,805–0,859) for marrow and 0 (0-0) for fibrous tissue. Bone tissue showed no significant differences.

In the peri-centric region the tissue fraction for PTH was 0,238 (0,211–0,276) for bone, 0,752 (0,724–0,785) for marrow and 0 (0–0,007) for fibrous tissue, as for control 0,223 (0,201–0,235) for bone, 0,777 (0,765–0,799) for marrow and 0 (0–0) for fibrous tissue.

Conclusion: In conclusion parathyroid hormone does not show significantly induced bone formation at a titanium alloy implant that has a porous coating of titanium alloy and inserted pressfit.

Purpose: Locked plating has become a commonly used technique in complex fracture and nonunion work. The combination of locked and unlocked screws in the same construct has been referred to as “hybrid” fixation. Little work is available to direct the specifics of this fixation method. The purpose of this study was to determine the relative contribution of the number and location of locked screws on the properties of hybrid plate constructs in an osteoporotic bone model.

Method: A prefabricated osteoporotic model was used for reproducibility (composite cylinders 35 mm in diameter and consisting of a 2.5 mm fiberglass shell filled with 10 lb/ft3 polyurethane). A 5mm gap model was used, and fixed with a 12 hole plate. Six different constructs were tested including 2 unlocked and 4 hybrid configurations. All screws were bicortical and placed with 4Nm of torque. Baseline removal (loosening) torque was recorded for each screw for comparison with removal torque after cyclic loading. Testing was performed with ±8Nm of torsional load and run to 100,000 cycles. Stiffness of each construct was measured at 10,000 cycle increments and the removal torque of each screw was recorded at the conclusion of the 100,000 cycles.

Results: Stiffness of the constructs was most affected by the number of screws. No effect was seen with the replacement of one or two unlocked screws with locked screws on each side of the gap. Replacement of three unlocked screws with locked screws increased the stiffness of the construct (p< 0.001).

Conclusion: At least three bicortical locked screws on each side of a construct are needed to increase the stiffness and decrease the loss of stiffness over 100,000 cycles of torsional stress in an osteoporotic surrogate model. Locked screws placed between the fracture and unlocked screws protect the unlocked screws from loosening and may have some clinical utility in fatigue of the construct.

Introduction: Revision hip implants have poorer clinical outcome than primary implants. The fixation of the implants is often compromised by the formation of an endosteal sclerotic bone rim during the process of aseptic loosening. The cracking procedure is a bone sparing, low energy surgical technique which produces a controlled local perforation of the sclerotic bone rim. In previous studies, we showed that fixation of revision implants significantly improved by the cracking technique for both titanium (Ti) and hydroxyapatite (HA) coated implants (1). In this study we compared the cracking technique with the common technique of reaming, which completely removes the sclerotic bone rim.

Methods: A paired animal study (n=10), in which revision cavities was created by 20 micromotion implant systems inserted in both knees. Micromotion was 0.5mm per gait cycle. After 8 weeks revision surgery was performed.

Crack revision: The splined crack tool was introduced over the implant piston with firm axial hammer blows. This producing controlled cracking and local perforation of the sclerotic endosteal rim. The tool is a 6.0 mm cylinder fitted with axially spaced 1.1 mm pointed splines (8.2 mm outer diameter).

Reaming revision: A flat bottomed reamer was inserted over the implant piston using one rotation per second. The outer diameter was 8.2 mm in order to remove the sclerotic bone rim. Stable revision Ti implants was inserted. Observation period was 4 weeks. Mechanical push-out tests were performed. Students’ paired t-test was used. Data presented as mean and SEM.

Results: Shear strength was markedly higher with the cracking procedure 1.33± 0.3MPa vs. 0.34 ± 0.2 MPa (p< 0.05). Similar results was seen for Stiffness 6.7± 2.0 vs. 1.6 ± 0.9 (p< 0.05) in favor of the cracking procedure. A non-significant increase was seen in energy absorption 170± 47 vs. 50± 29 (p=0.07).

Discussion: The cracking procedure improves the mechanical fixation of Ti revision implants compared to a reaming procedure. Shear strength and stiffness was consistently higher for all implant pairs. We have previously shown that the sclerotic bone rim is a barrier for bone ingrowth and that implants inserted with an intact sclerotic bone rim will have a poor biomechanical fixation. Additionally, revision implants inserted with the cracking technique obtained a mechanical fixation comparable to primary implants. Reaming procedures are often used in hip revision surgery. However, as loss of bone stock is a common feature of revision cavities, the reaming procedure may not always be an optimal preparation method of the bone. Excessive removal of bone by reaming may compromise the long term implant stability or increase the risk of peri-implant fractures. The cracking technique may be an alternative or supplemental procedure to reaming.

Aims: Early bone ingrowth is known to increase primary implant þxation and reduce the risk of early implant failure. RGD peptide (Arg-Gly-Asp) has been identi-þed as playing a key role in osteoblast attachment and proliferation on various surfaces. The aim of this study is to test whether a monolayer of RGD peptide on Ti implants will increase bone ingrowth in vivo. Methods: Controlled canine study (n=8). 6 x 10mm plasma sprayed porous coated implants (Ti6Al4V) was inserted as press-þt in the proximal tibia bilaterally. Observation period was 4 weeks. Implants was coated in a 100 μM solution of cylic (RGDfK) peptide for 24 hours (Biomet-Merck, Darmstadt, Germany). Two dogs had to be excluded due to wrong placement of the implants. Results are presented as median and range. Results: A signiþcant increase in bone/implant contact was seen for the RGD treated group (p< 0.05). Bone fraction at the interface was 0.18 (0.10–0.45) compared to 0.09 (0.05–0.14) for the control. Mechanical þxation, measured by push-out test, was increased. Shear strength was 85% higher for the RGD group; however this difference was not signiþcant. Conclusions: This study shows that implant surface treatment with RGD enhances early bone ingrowth to press-þtted implants. However, future studies will be preformed regarding coating integrity and long-term effects, as well as its performance under loaded conditions.

Aim: The objective of this study was to compare the initial stability of an uncemented curved long stem femoral component (ABR, Zimmer, Warsaw Indiana) implanted using conventional broaching to an uncemented ABR implanted as a revision using morselized compacted allograft to fill a circumferential proximal bone defect.

Materials and Methods: Primary: Eight fresh frozen human femurs were implanted with correctly sized ABR stems without cement using standard surgical instruments and 1mm distal overreaming. The implant was loaded cyclically at 3 hz on a MTS servohdraulic materials test frame in a direction representing the peak force of the stance phase of the gait (2.5 X body weight). A 3-dimensional motion measurement device with six linear variable transformers measure the relative motion between the bone and the prosthesis with an accuracy of 0.5μm. The contruct was loaded in ten cycle increments until the component was seated and then recoverable motion was recorded over a further ten loading cycles. Revision: In the revision case bone loss was modelled by removing all the cancellous bone from the proximal femur with 1mm distal overreaming. Morselized bone graft was impacted into the defect using specially prepared smooth tamps shaped to the geometry of the ABR stem. The original prosthesis was then reimplanted without the use of cement. The seating and testing cycles were repeated. Primary and revision vectors were compared using a paired students t – test.

Results: There was no statistically significant difference in the magnitude of the resultant vector of the 3 translational components of the micro motion between the two methods of implantation (p=0.19).

Conclusions: Initial stability of a cementless femoral implant is a requirement for bone ingrowth. The use of compacted morsellized allograft in a revision Total Hip Arthroplasty with a proximal circumferential defect can provide a stable bed for the implantation of an uncemented femoral component. The revised implant using the compacted allograft method was initially as stable as the primary implantation.

This technique would be particularly applicable when the surgeon would like to avoid the use of cement in a revision setting. The use of this method for uncemented revision Total Hip Arthroplasty should be studied further in a clinical setting before it is advocated for widespread use.