Joan E. Bechtold

Reduction in cement-bone interface shear strength between primary and revision arthroplasty

This study quantified changes in the cement-bone interface shear strength between primary and first- and second-revision arthroplasties as a function of mechanical interlock between the cement and bone. There were 128 segments obtained from four pairs of fresh human femora that were prepared sequentially as for primary and first and second revisions, taking care to maintain original canal morphology. Cement was pressurized into the cavity of the anatomic specimens, and the maximum interface shear strength between the cement plug and the bone was experimentally determined for each revision. First-revision interface shear strength was reduced to 20.6% of primary strength, and second revision strength to 6.8% of primary strength.

Patellar prosthesis positioning in total knee arthroplasty. A roentgenographic study.

Patellar prosthesis positioning in 40 primary total knee replacements was evaluated with regard to (1) patellar tilt, (2) angle between the patellar component and patellar bony remnant, (3) lateral versus medial placement, (4) patellar height, and (5) size of the patellar component versus patellar length. Two different designs were evaluated. Twenty knees that were resurfaced with a patellar button prosthesis and implanted with conventional surgical technique constituted Group A. Twenty knees that were resurfaced with a new biconvex prosthesis and implanted with specially designed instrumentation constituted Group B. For Group A, the patellar tilt averaged 4.25 degrees preoperative and 8.35 degrees postoperative. For Group B the patellar tilt averaged 4.60 degrees preoperative and 1.83 degrees postoperative. In Group B, there was significant improvement in patellar position, with only two patients showing values exceeding the neutral range (p = 0.0409). The angle between the patellar component and the bony remnant in Group A averaged 5.975 degrees, and in Group B, 1.447 degrees. Two patients in Group A and 18 patients in Group B had values in the normal range for patellar tilt and the angle between the prosthesis and the bone. The prosthesis was inserted medial to the patellar center in six patients in Group A and one patient in Group B. A significant decrease in patellar height was noted for both groups, but values were still within the normal range. Both groups showed significant increase in the postoperative articular length, indicating that oversized prostheses were inserted and excess bone removed.

Forces required to initiate sliding in second-generation intramedullary nails

Second-generation intramedullary nails, which allow the fixation screw that is placed in the femoral head to slide distally and thus allow compression of the fracture of the femoral neck, have become a popular option for the treatment of ipsilateral fractures of the femoral neck and shaft. However, the sliding characteristics of the screw within the barrel of the nail or the side-plate have not been assessed biomechanically, to our knowledge. The goal of the current study was to investigate the forces required to initiate sliding of the proximal screw in intramedullary devices and to compare these forces with those required to initiate sliding of hip screws. The loading configuration simulated the typical angle of 135 degrees between the intramedullary nail and the proximal screw. The forces required to initiate sliding of the proximal screw, with the screw extended fifty-one, seventy-six, eighty-six, and 102 millimeters beyond the proximal end of the barrel, were measured for three different types of second-generation intramedullary nails (Recon, ZMS, and Gamma), a sliding compression hip screw, and an intramedullary hip screw, and these forces were then compared. With each amount of extension of the screw, the hip screws required lower forces to initiate sliding than did the second-generation intramedullary devices. Of the second-generation devices, the Gamma nail required the highest forces to initiate sliding; the Recon and ZMS nails required 20 to 40 percent lower forces compared with the Gamma nail. None of the devices jammed in any of the loading configurations that were tested. When the extension of the screw was increased, higher forces were required to initiate sliding. CLINICAL RELEVANCE: Since sliding allows continued compression of the fracture, surgeons should be aware that, compared with hip screws, the second-generation nails that we tested required higher loads to initiate sliding and to generate subsequent compression of the fracture.

Loading improves anchorage of hydroxyapatite implants more than titanium implants.

Roentgen Stereophotogrammetric Analysis (RSA) studies have shown that the quality of the early fixation of implants has a dominant effect on their long-term function. To evaluate methods to improve their fixation, we examined the influence of mechanical loading and surface coating on the quality of the bone-implant interface. We compared the fixation of a cylindrical, stable 6.0 mm implant initially surrounded by a 0.75 mm concentric gap, after 4 weeks of loaded or unloaded conditions. Two types of surfaces were analyzed: plasma sprayed hydroxyapatite (HA) and plasma sprayed titanium (Ti). The histomorphometric evaluation showed that HA implants had greater bone coverage than Ti implants, and this coverage was further increased under loaded conditions only for HA. Furthermore, loading reduced the fibrous tissue coverage for the HA implants, while it increased fibrous tissue coverage for Ti implants. These findings were in agreement with pushout results showing that HA implants had greater shear strength, stiffness, and energy than Ti implants, and (except for energy) these parameters were further increased under loaded conditions only for HA. In addition, because the two implant surfaces exhibited a different relative response to load, it is important to evaluate new surfaces under the more clinically relevant loaded condition.

Local Bisphosphonate Treatment Increases Fixation of Hydroxyapatite-Coated Implants Inserted with Bone Compaction

It has been shown that fixation of primary cementless joint replacement can independently be enhanced by either: (1) use of hydroxyapatite (HA) coated implants, (2) compaction of the peri‐implant bone, or (3) local application of bisphosphonate. We investigated whether the combined effect of HA coating and bone compaction can be further enhanced with the use of local bisphosphonate treatment. HA‐coated implants were bilaterally inserted into the proximal tibiae of 10 dogs. On one side local bisphosphonate was applied prior to bone compaction. Saline was used as control on the contralateral side. Implants were evaluated with histomorphometry and biomechanical push‐out test. We found that bisphosphonate increased the peri‐implant bone volume fraction (1.3‐fold), maximum shear strength (2.1‐fold), and maximum shear stiffness (2.7‐fold). No significant difference was found in bone‐to‐implant contact or total energy absorption. This study indicates that local alendronate treatment can further improve the fixation of porous‐coated implants that have also undergone HA‐surface coating and peri‐implant bone compaction.

Soaking morselized allograft in bisphosphonate can impair implant fixation

The use of impacted, morselized allograft is a well-established way to provide initial stability of revision joint replacements. We investigated whether rinsing morselized allograft in bisphosphonate and subsequently impacting it around experimental titanium-coated implants would further facilitate biomechanical implant fixation and graft incorporation. In 10 dogs, a pair of titanium implants surrounded by a 2.5-mm gap was inserted into the proximal part of each humerus during two separate surgeries to allow two observation periods. The gap was filled with impacted, morselized allograft soaked in either bisphosphonate (alendronate, 2 mg/mL) or saline (control). Unbound alendronate was not rinsed away. During the first surgery, one pair of implants (alendronate and control) was inserted into one humerus. Eight weeks later, a second pair of implants was inserted into the contralateral humerus. The first pair of implants was observed for 12 weeks and the second pair for 4 weeks. Implants were evaluated by histomorphometry and biomechanical pushout test. We found substantially decreased biomechanical implant fixation for all implants surrounded by impacted, morselized allograft that had been soaked in alendronate. Furthermore, the alendronate treatment blocked formation of new bone and inhibited resorption of the graft material. Although limited by the specific dose of alendronate used and the omission of rinsing away excess bisphosphonate, this study warrants caution and calls for further experimental research before impacting alendronate-soaked morselized allograft around clinical joint replacements.

The effects of freeze-drying and ethylene oxide sterilization on the mechanical properties of human patellar tendon.

Human patellar tendon allografts are used to replace injured anterior cruciate ligaments. They are often stored by freeze-drying or freezing before use and have been sterilized by gamma irradiation or ethylene oxide gas. Studies comparing the mechanical properties of tendons preserved by freeze-drying and sterilized by ethylene oxide have yielded conflicting results. This study examined the effects of freeze-drying and ethyl ene oxide sterilization on the mechanical properties of human patellar tendons obtained from 11 human ca davers ; all were free of musculoskeletal disease. The 22 patellar tendons were halved longitudinally; 7 were fro zen at -70°C for 8 weeks and served as controls; 8 were freeze-dried; and 7 were freeze-dried and sterilized with ethylene oxide gas. Ultimate tensile stress and longi tudinal strain were greater for frozen than for freeze- dried, but not greater than freeze-dried, ethylene oxide- sterilized tendons (P < 0.05). The modulus of elasticity was similar in all tendon groups. The study suggests that some in vitro mechanical properties of frozen ten dons may be marginally superior to the properties of freeze-dried tendons, even after correction for size dif ference. Ethylene oxide gas sterilization had no adverse effect on tendon mechanical properties.

Effects of short-term alendronate treatment on the three-dimensional microstructural, physical, and mechanical properties of dog trabecular bone

The bisphosphonate, alendronate, is well known for its potent inhibition of osteoclast-mediated bone resorption. It has been used clinically for the treatment of osteoporosis and has also recently been used to reduce osteolysis around prostheses in a canine revision model of implant loosening (femoral condyle). In this study, the effects of alendronate on trabecular bone properties were assessed in dogs at an oral dose of 0.5 mg/kg per day over a 12 week period, and compared with control dogs. Cubic cancellous bone specimens were produced from lumbar vertebrae (L-1 and L-2) and bilateral proximal humeri. These specimens were scanned using a high-resolution microcomputed tomography (micro-CT) system. From accurate data sets, three-dimensional microstructural properties were calculated and physical and mechanical properties were determined. Treatment with alendronate increased bone volume fraction by 9.5%, 7.7%, 7.4%, and 18.4%, respectively, in L-1, L-2, humeral greater tuberosity, and humeral head trabecular bone. In the lumbar vertebrae, the alendronate-treated trabeculae were thicker and lower in bone surface-to-volume ratio. In the greater tuberosity, the alendronate-treated trabeculae were thicker, lower in bone surface-to-volume ratio, and less anisotropic. In the humeral head, the alendronate-treated trabeculae were thicker, less anisotropic, lower in surface density, and showed decreased trabecular separation. Alendronate significantly increased apparent density and collagen density in the lumbar vertebrae and humeral heads, and significantly decreased collagen concentration in the vertebrae. In the lumbar vertebrae, Youngs modulus in the cephalocaudal direction, ultimate stress, and failure energy were significantly increased in the alendronate-treated group. The changes in mechanical properties in the humeral head trabecular bone were similar to those seen in the lumbar vertebrae. Our results demonstrate that alendronate increases the mechanical properties of healthy canine trabecular bone after short-term treatment. The physical and microstructural changes of trabecular bone are consistent with the significantly increased mechanical properties.

The effect on bone growth enhancement of implant coatings with hydroxyapatite and collagen deposited electrochemically and by plasma spray.

Skeletal bone consists of hydroxyapatite (HA) [Ca(10)(PO(4))(6)(OH)(2)] and collagen type I, both of which are osseoconductive. The goal of osseointegration of orthopedic and dental implants is the rapid achievement of a mechanically stable long-lasting fixation between bone and an implant surface. In this study, we evaluated the mechanical fixation and tissue distribution surrounding implants coated with three surfaces: plasma-sprayed HA coating, thinner coating of electrochemical-assisted deposition of HA, and an identical thin coating with a top layer of mineralized collagen. Uncoated plasma-sprayed titanium (Ti-6Al-4V) served as negative control. The electrochemical-assisted deposition was performed near physiological conditions. We used a canine experimental joint replacement model with four cylindrical implants (one of each treatment group) inserted in the humeri cancellous metaphyseal bone in a 1 mm gap. Observation time was 4 weeks. The mechanical fixation was quantified by push-out test to failure, and the peri-implant tissue formation by histomorphometric evaluation. HA coatings deposited by plasma spray technique or electrochemically, increased the mechanical fixation and bone ongrowth, but there was no statistical difference between the individual HA applications. Addition of collagen to the mineralized phase of the coating to create a more bone natural surface did not improve the osseoconductive effect of HA.

Comparison of joint position sense after total knee arthroplasty

A comparison of joint position sense, determined by reproducibility of index angles and their subsequent change, was performed in 55 knees that had undergoing a semiconstrained total knee arthroplasty. Knees were stratified into groups that represented arthroplasties performed with or without posterior cruciate ligament retention, with or without resurfacing of the patella, and with or without cement for fixation. There was no significant difference in joint position sense among all the arthroplasty groups. Likewise, there was no difference in joint position sense between any of the arthroplasty groups and an age-matched control group of 32 knees in 32 patients who had not previously undergoing a total knee arthroplasty. Knee arthroplasty does not affect joint position sense.