[Supportive drugs for improved implant healing]. / Medikamentöse Verbesserung der Implantateinheilung.

BACKGROUND: Improvement of implant healing in orthopedic and trauma surgery serves to improve the life expectancy of the implant. Good primary stability by clamping is a prerequisite for secondary stability and for the actual integration and healing of the implant. RESULTS: Possible causes of implant loosening are abrasive particles, which arrive at non-integrated implants at the unsealed prosthesis-bone interface and provoke a macrophage-mediated foreign body reaction, resulting in periprosthetic osteolysis. Numerous animal studies have already described the use of bisphosphonates to inhibit osteolysis induced by abrasion and secondary instability. In patients with total knee arthroplasty, a decrease in prosthetic migration under the influence of bisphosphonates could be shown. The stimulation of bone formation around the implants and the resulting implant healing was demonstrated both in animal experiments for bone morphogenetic proteins (BMP) and in case reports for intermittent parathyroid hormone administration. CONCLUSION: By using supportive drugs, it is possible to achieve an improvement in the osseointegration of implants; thus, more rapid secondary stability and load-bearing are expected.

[Hybrid and uncemented hip arthroplasty: Contribution margin in the German lump sum reimbursement system]. / Teilzementierte und unzementierte Hüftendoprothetik: Deckungsbeitrag im deutschen Fallpauschalensystem.

BACKGROUND: The economization of inpatient care began when lump sum reimbursement was introduced into the hospital sector. Since then, total hip arthroplasty (THA) has experienced a rapid development in terms of annual procedures and the optimization of the clinical pathway. Therefore, it is obvious to highlight THA as one of the most common procedures in the German health care system. In this work, the two most common techniques for the fixation of THA are investigated with regard to their cost structure and their influence on the clinical result. OBJECTIVES: In Germany, uncemented and hybrid fixation are used for THA. In this study we investigated the differences in material costs, the duration of surgery, and the length of stay. MATERIALS AND METHODS: For each fixation technique a retrospective cost analysis was carried out, based on the data from the treatment documentation of the University Hospital for Orthopedics, Rostock (OUK). The mean values of the parameters and expenses have been reviewed with descriptive statistics for differences. RESULTS: With regard to total costs and the contribution margin there was no statistically significant difference. CONCLUSIONS: Although there are differences in individual cost areas, in total costs, cost advantages and disadvantages cancel each other out. Thus, from an economic perspective no particular technique can be recommended.

[Economic impact of infected total hip arthroplasty in the German diagnosis-related groups system]. / Ökonomische Auswirkungen infizierter Hüfttotalendoprothesen im DRG-System.

BACKGROUND: The treatment of an infected total hip arthroplasty is becoming an increasing economic problem. The additional costs of treatment are insufficiently represented in the diagnosis-related groups (DRG) categories. The aim of this study was to clarify whether the costs can be covered under the German DRG system and to identify the extent of the surplus or negative balance. PATIENTS AND METHODS: A retrospective analysis of the treatment costs of total hip arthroplasty was carried out. Data from all patients treated at the orthopedic clinic of the University Hospital in Rostock were collected from patient records and from the hospital information system and calculation of the personnel and material costs using data from the central pharmacy and control centre of the University of Rostock. RESULTS: In this study a total of 49 patients were included. The average treatment costs were 29,331.36 EUR per patient for an infected and 6,263.59 EUR for a primarily non-infected total hip arthroplasty. A comparison between the calculated and compensated costs resulted in an average deficit of 12,685.60 EUR per patient and an average surplus of 781.41 EUR per patient in the control group. CONCLUSIONS: An economically viable treatment of infected total hip arthroplasty was not possible mostly due to the increased personnel and material costs but also to the lack of inclusion of the procedures in the DRG system. Further multicenter cost analysis studies and extensive quality assurance measures are necessary with respect to a comprehensive medical standard for a medically meaningful and economically reasonable treatment of periprosthetic infections.

[Biphosphonate-induced femoral stress fractures : A new problem and knowledge so far - case report]. / Bisphosphonatinduzierte Stressfrakturen am Femur : Ein neues Problem und bisherige Erkenntnisse - Fallbericht.

During the last several years the treatment of osteoporosis with bisphosphonates has become accepted as a safe and effective procedure. However, recently there have been increasing numbers of reports of rare complications in the literature. Particularly the occurrence of atypical fractures of the femur has become a focus of interest but the problem is insufficiently known and only rarely addressed in the scientific discussion. The case illustrated here and a survey of the important facts in the recent literature highlight essential aspects of long-term bisphosphonate therapy.

[Periprosthetic infections following total hip replacement with ESBL-forming bacteria: importance for clinical practice]. / Periprothetische Infektionen nach Hüfttotalendoprothese mit ESBL-bildenden Bakterien: Bedeutung für die klinische Praxis.

Implant infections remain feared and severe complications after total hip replacement. An even higher rate of periprosthetic infections can be observed after revision surgery in comparison to primary total hip replacement. An additional threat for patients with artificial joints arises from the fact that bacteria resistant to a multitude of antibiotics are encountered with increasing frequency in the hospital setting.Among these the enterobacteria producing extended spectrum ß-lactamases (ESBL) are the second most frequent group of multiresistant pathogens. ESBLs are enzymes which possess the ability to hydrolyse third and fourth generation cephalosporins resulting in a distinctive resistance against these antibiotics. Even though ESBLs were first described in the early 1980's and now represent pathogens of utmost importance in intensive care units, they have been hardly considered in orthopedic and trauma surgery.In the present manuscript we provide an overview of the epidemiology and diagnostics of ESBL-expressing bacteria and demonstrate the difficulties in managing implant-associated infections with resistant bacteria. Furthermore, we emphasize the importance of recognizing ESBL-positive bacteria as increasingly important pathogens which require special precautions and treatment. Clinical evaluations suggest that ESBLs in orthopedic and trauma surgery are not a rare phenomenon any more.

Bone remodeling in proximal HA-coated versus uncoated cementless SL-Plus((R)) femoral components: a 5-year follow-up study.

BACKGROUND AND PURPOSE: Bone resorption at the femoral stem due to stress shielding has been particularly observed secondary to cementless hip replacement. This prospective study of 126 total-hip replacements was performed to examine clinical outcomes and changes in peri-prosthetic bone density after implantation of a double-tapered cementless femoral component manufactured with versus without hydroxyapatite (HA) coating. METHODS: Sixty-seven femoral components with and 59 femoral components without proximal HA coating were implanted and examined after a mean follow-up of 5.8 (SD 1.1, 2.8-7.8) years. The Harris hip score (HHS) and plain radiographs were used for clinical and radiological follow-up evaluations. Possible changes in peri-prosthetic bone mineral density (BMD) were measured by dual-energy X-ray absorptiometry (DEXA). RESULTS: Clinical outcome, measured by HHS, was similar in both groups. On plain radiographs, significantly less radiolucent lines were observed for the coated implants. DEXA revealed a significant increase in BMD at the proximal zones, along the medial side, and at the stem tips in the coated compared with the non-coated implants. INTERPRETATION: Hydroxyapatite-coated implants yield favorable radiographic characteristics, but no greater clinical benefit after 5 years' implantation.

Effect of alendronate and intermittent parathyroid hormone on implant fixation in ovariectomized rats.

BACKGROUND: Intermittent administration of parathyroid hormone (PTH) leads to bone formation by increasing osteoblast numbers and activity levels. Animal studies have shown that intermittent PTH administration increases implant fixation in normal rats. The purpose of this study was to analyze the osseous incorporation of an implant in osteoporotic rats while treating them with intermittent PTH (1-34) or alendronate. METHODS: A total of 36 ovariectomized (OVX) Wistar rats were randomized into three groups. Polymethylmethacrylate cement rods were implanted in one tibia in each rat. The three groups received daily PTH (60 mug/kg body weight [BW]), alendronate (200 mug/kg BW), or saline (0.5 ml/kg BW). A sham-ovariectomized group (n = 12) was treated with saline. After 2 weeks, the area around the implants was analyzed by histomorphometry for bone volume density (BVD) and implant bone contact. Bone mineral density (BMD) was evaluated by dual-energy X-ray absorptiometry. RESULTS: The BVD was higher in the specimens treated with PTH than in the other groups. PTH improved the BVD, BMD, and implant bone contact. Alendronate doubled the implant bone contact compared to the OVX and sham groups but did not improve BVD or BMD. CONCLUSIONS: These findings confirm that intermittent PTH enhances implant fixation in osteoporotic bone. The clinical significance of these findings is that application of intermittent PTH may be beneficial for early implant fixation in fractures, nonunions, and prosthetic replacements when bone density is decreased.

Bisphosphonates can block the deterioration in implant fixation after withdrawal of intermittent doses of parathyroid hormone.

We have examined the deterioration of implant fixation after withdrawal of parathyroid hormone (PTH) in rats. First, the pull-out force for stainless-steel screws in the proximal tibia was measured at different times after withdrawal. The stimulatory effect of PTH on fixation was lost after 16 days. We then studied whether bisphosphonates could block this withdrawal effect. Mechanical and histomorphometric measurements were conducted for five weeks after implantation. Subcutaneous injections were given daily. Specimens treated with either PTH or saline during the first two weeks showed no difference in the mechanical or histological results (pull-out force 76 N vs 81 N; bone volume density 19% vs 20%). Treatment with PTH for two weeks followed by pamidronate almost doubled the pull-out force (152 N; p < 0.001) and the bone volume density (37%; ANOVA, p < 0.001). Pamidronate alone did not have this effect (89 N and 25%, respectively). Thus, the deterioration can be blocked by bisphosphonates. The clinical implications are discussed.

Attachment of PMMA cement to bone: force measurements in rats.

The attachment of an implant material to bone relates to surface roughness and surface chemistry. There is a relatively low chemical bonding strength of so-called bioactive surfaces. Hydroxyapatite interfaces typically have an interfacial tensile strength of 0.15-1.5 MPa. An attachment force similar to that of bioactive surfaces might also be reached through mechanical interlock with ordinary bone cement. This study measured bone cement interfacial tensile strength for polished (R(a) 0.5 microm) and regular (R(a) 4.8 microm) vacuum mixed PMMA bone cement. Bone bonding was evaluated by a detachment test. We used unloaded cement surfaces, which could be detached from the bone. Titanium plates were developed such that a cement fill was contained within a plate, which was contained within a titanium holder. The cement surface came into contact with traumatized bone only, and the rest of the plate had no contact with tissue. The cement surface was either polished or left untreated after conventional preparation. Four weeks later, the plates were detached from the bone by a perpendicular force. The detaching load of the polished cement surface never exceeded 0.07 MPa, whereas for unpolished cement there was a load up to 0.9 MPa. The results suggest that surface irregularities and microinterlock enable an attachment that can resist tension between bone and a cement surface.

Pressure-induced periprosthetic osteolysis: a rat model.

Recent animal experiments have indicated that oscillating fluid pressure at the interface of bone and implant can lead to osteolysis. However, external nonphysiologic saline solutions were used to generate the pressure in these studies. In the present study on 15 Sprague-Dawley rats, hydrostatic pressure fluctuations were applied to bone through body fluids, by compressing a soft-tissue membrane adjacent to the proximal tibia. A titanium plate was fixed to the bone surface. After 28 days of osseointegration of the plate, a 1-mm-wide gap was created between it and the cortical bone and 5 days were given for fibrous tissue to form. Load was transmitted to this soft tissue by applying force on a piston mounted in the plate. In six rats, a cyclic pressure of 0.6 MPa was then applied to this tissue by 20 cycles twice a day with a frequency of 0.17 Hz for 5 days. The remaining rats served as controls, with the piston left untouched in its upper position. All of the rats were killed 10 days after creation of the gap. Histological sections were produced at a right angle to the loaded surface. In the pressurized specimens, osteoclastic bone resorption was dramatic. In all specimens, the original cortex was almost entirely resorbed but new woven bone had formed deeper in the marrow and walled off a cystic lesion. When necrotic remnants of the cortex were still in place, new woven bone was seen on the side away from the piston. This "lee effect" may indicate that bone formation was inhibited by fluid flow away from the pressurized tissue. The specimens with a nonloaded piston showed no signs of resorption. This new experimental model shows again that a moderate rise of hydrostatic pressure at the interface of bone and implant leads to considerable bone resorption. This could be a mechanism of prosthetic loosening.

Intermittent parathyroid hormone (1-34) enhances mechanical strength and density of new bone after distraction osteogenesis in rats.

Distraction osteogenesis is used both for leg lengthening and for bone transportation in the treatment of fractures and nonunions. The main problem with this method is that the time until full recovery may be up to a year, partly because of the time needed for the new formed bone to consolidate and become strong enough for weight bearing. We have studied whether intermittent parathyroid hormone (PTH(1-34)) could accelerate the consolidation of new formed bone after distraction osteogenesis in rats. Forty-seven, 3-months-old male Sprague-Dawley rats underwent lengthening of the right femur using an external fixator. After a middiaphyseal osteotomy and a 7-day latency period, the callus was distracted during 10 days, with a distraction rate of 0.25 mm twice a day. The consolidation time was either 20 days or 40 days after distraction was completed. A dose of 60 microg of human PTH(1-34)/kg body weight/injection or vehicle was given every second day beginning 30 days before the rats were killed. Both femura of each rat were subjected to mechanical testing and dual-energy X-ray absorptiometry. Blinded histological examination was done for the distracted femura. In the 20 days consolidation experiment, PTH(1-34) increased ultimate load (56%), stiffness (117%), total regenerate callus volume (58%), callus BMC (24%) and histologic bone density (35%) compared to untreated distraction osteogenesis specimens. In the 40 days consolidation experiment, PTH(1-34) increased ultimate load (54%), stiffness (55%), callus BMC (33%) and histologic bone density (23%) compared to untreated distraction osteogenesis specimens. Total regenerate callus volume was unchanged. The contralateral femur also became stronger, stiffer and denser under PTH(1-34) treatment, but to a lesser degree. PTH(1-34) might become useful to shorten the consolidation time after distraction osteogenesis in humans.

Implant fixation enhanced by intermittent treatment with parathyroid hormone.

The intermittent administration of parathyroid hormone (PTH) increases the formation of bone by stimulating osteoblastic activity. Our study evaluates the possibility that intermittent treatment with PTH (1-34) may also enhance the implant-bone fixation of stainless-steel screws. Twenty-eight rats received one screw in either one (n = 8) or in both (n = 20) proximal tibiae. We administered either PTH (1-34) in a dosage of 60 microg/kg/day (n = 14) or vehicle (n = 14) over a period of four weeks. At the end of this time, the degree of fixation was assessed by measuring the removal torque on one screw in each rat (n = 28) and the pull-out strength on the contralateral screw (n = 20). PTH increased the mean removal torque from 1.1 to 3.5 Ncm (p = 0.001) and the mean pull-out strength from 66 to 145 N (p = 0.002). No significant differences in body-weight or ash weight of the femora were seen. Histological examination showed that both groups had areas of soft tissue at the implant-bone interface, but these appeared less in the PTH group. These results indicate that intermittent treatment with PTH may enhance the early fixation of orthopaedic implants.

Parathyroid hormone (1-34) increases the density of rat cancellous bone in a bone chamber. A dose-response study.

Intermittent treatment with parathyroid hormone I(PTH) has an anabolic effect on both intact cancellous and cortical bone. Very little is known about the effect of the administration of PTH on the healing of fractures or the incorporation of orthopaedic implants. We have investigated the spontaneous ingrowth of callus and the formation of bone in a titanium chamber implanted at the medioproximal aspect of the tibial metaphysis of the rat. Four groups of ten male rats weighing approximately 350 g were injected with human PTH (1-34) in a dosage of 0, 15, 60 or 240 microg/kg/day, respectively, for 42 days from the day of implantation of the chamber. During the observation period the chamber became only partly filled with callus and bone and no difference in ingrowth distance into the chamber was found between the groups. The cancellous density was increased by 90%, 132% and 173% in the groups given PTH in a dosage of 15, 60 or 240 microg/kg/day, respectively. There was a linear correlation between bone density and the log PTH doses (r 2= 0.6). Our findings suggest that treatment with PTH may have a potential for enhancement of the incorporation of orthopaedic implants as well as a beneficial effect on the healing of fractures when it is given in low dosages.

Low-grade infection after a total knee arthroplasty caused by Actinomyces naeslundii.

Here, we present a case of an 85-year-old woman with a low-grade-infection caused by Actinomyces naeslundii after total-knee arthroplasty (TKA) followed by septic loosening. Actinomyces naeslundii was cultured from a tissue sample from the knee joint capsule/synovial tissue obtained after the initial TKA. A review of the literature revealed two cases of periprosthetic infection and another three cases of arthritis due to Actinomyces naeslundii. So far, no standard treatment for periprosthetic infections caused by Actinomyces species has been established.

[Cementless femoral neck prosthesis CUT--clinical and radiological results after 5 years]. / Die zementfreie Schenkelhalsprothese CUT--klinisch-radiologische Ergebnisse nach 5 Jahren.

AIM: With regard to total replacement of the hip, revision arthroplasty poses a challenge especially for younger patients. In spite of substantial improvements, new materials and operation techniques are still not able to prevent a shorter running life of prostheses in this group. The present work aims at evaluating clinical and radiological mid-term results of the femoral neck prosthesis CUT (ESKA Orthodynamics Lübeck) to answer the question of whether this implant is recommendable for younger patients. METHOD: Between 2001 and 2005 a consecutive series of 99 CUT prostheses was performed in 86 patients (50 female, 36 male) with a mean age of 50 (17-72) years and again evaluated clinically and radiologically after 5.4 (1.7-6.5) years. 84 cases were operated using a posterior approach and 15 cases were operated according to the anterolateral Watson-Jones approach. For clinical evaluation the Harris hip score and the visual analogue scale (VAS) for pain measurement were applied. Standard anteroposterior radiographs of the pelvis and lateral radiographs of the operated hip were compared to radiographs taken in the recovery room by two independent observers. Interobserver measurement discrepancy of the implant angle was 2.6 +/- 1.4 degrees. With 4 degrees being the maximum discrepancy, it was defined as the threshold of the normal range of 145 degrees (141-149 degrees). Additionally, the amount of femoral neck resection, the contact of the medial corticalis with the proximal stem, and the contact of the lateral corticalis with the distal part of the stem, periarticular ossifications and stable fixation by bone ingrowth according to Engh et al. were evaluated. Five delineated sections around the femoral component for evaluation of looseness or progressive loosening were used according to Gruen et al. Radiological evaluation of the cup was performed according to Charnley and DeLee. RESULTS: The survival rate according to Kaplan-Meier was 98 % after on average 6.6 years. The Harris hip score significantly improved from 50 (16-83) points preoperatively to 98 (40-100) points at the time of follow-up (p < or = 0.05). 82 % achieved an excellent result (91-100 points), 10% a good (91-90 points), 4% had a moderate (71-80 points) and 4% had a bad (< 70 points) result. Six prostheses had to be revised. One of them had to be changed to a cementless standard stem after 5 years because of aseptic loosening. Another one had to be revised after 2.7 years because of chronic thigh pain. Two painful hips had a capsular revision. In one case the liner had to be changed and one case had an exchange of the femoral ball for a better femoral offset. The VAS revealed a significant reduction of pain in rest and under load (p < or = 0.05). 92% had a correct subcapital neck resection. The recommended implant angle of 145 degrees was seen in 72% while a valgus alignment in 18% and a varus alignment in 10% was measured. Undersizing of the CUT-prosthesis was seen in 27 cases. Nine of these cases developed a varus alignment. Osseous integration of the cup and stem was seen in 100% and in 95%, respectively. CONCLUSION: This study demonstrates the CUT prosthesis as a bone-preserving prosthesis with good functional and radiological results and therefore as an alternative joint replacement in younger patients.

Tensile bond between bone and titanium: a reappraisal of osseointegration.

When Branemark in the 1970s established the term osseointegration, this implied a direct chemical bond between a titanium implant and bone. However, ultrastructural studies seemed not to support this idea, and osseointegration came to be defined as the absence of interfering fibrous tissue. Titanium was therefore described as bioinert rather than bioactive. We now demonstrate mechanically a chemical bond between bone and titanium, using unloaded cp titanium plates, similar to those used in previous studies on prosthetic loosening. Tensile force can be transmitted only by chemical bonds. Bone-bonding was therefore evaluated by a detachment test. The plates were developed so that a flat titanium surface touched traumatized bone and the rest of the detachable part had no contact with surrounding tissue. The titanium plates were either polished and sterilized in an autoclave or treated in 4 M NaOH and then heated to 600 degrees C according to Yan et al. (1996). After 4 weeks, the plates were separated from the bone by a perpendicular traction force. The detaching load of the untreated titanium plates never exceeded 0.03 MPa, whereas with treatment it increased to median 0.8 MPa, with bone remaining attached to parts of the plates after detachment. Our findings confirm that a chemical bond can be obtained within 4 weeks with the described pretreatment. It may occur also without treatment, after a longer implantation time.

Early effect of parathyroid hormone (1-34) on implant fixation.

Intermittent systemic administration of parathyroid hormone increases bone formation by stimulating osteoblastic activity. The current study determined how parathyroid hormone (1-34) administration influences the bony fixation of stainless steel screws with time. A screw was implanted in the left tibia and a metal rod was implanted in the right tibia in 30 adult male rats that then were injected three times a week with human parathyroid hormone (1-34) at 60 microg/kg/injection (n = 15) or saline (n = 15). The animals were euthanized after 1, 2, or 4 weeks of treatment. Eight additional rats received only the screw and were euthanized immediately after implantation. No significant effects of parathyroid hormone on body weight change or ash weight of the femurs were seen. The degree of fixation was assessed by measuring pullout strength of the screws. The mean pullout strength immediately after implantation was 12 N. The pullout strength of the group injected with saline was 33 N after 1 week, 23 N after 2 weeks, and 41 N after 4 weeks. The pullout strength of the group injected with parathyroid hormone increased to 43 N after 1 week, 58 N after 2 weeks, and 100 N after 4 weeks. The increase at 2 and 4 weeks was statistically significant. Strength reflects the mechanical properties of the bone within the screw threads. The contralateral tibia with its metal rod was used for blinded histologic assessment. Parathyroid hormone increased the fraction of the metal surface having contract with bone without an intervening soft tissue layer from 45% to 69% after 1 week. The current results suggest that intermittent parathyroid hormone treatment can enhance early implant fixation by enhancing the density of the surrounding bone and by increasing the implant bone contact.

Parathyroid hormone (1-34) increases attachment of PMMA cement to bone.

The attachment of an implant material to bone is related to the surface of the implanted material and the ability of the bone to form around the implant. Intermittent parathyroid (PTH) administration increases bone formation by stimulating osteoblastic activity. Little is known about the effect of PTH administration on orthopedic implant incorporation. The present study determined how PTH (1-34) administration influenced bone bonding, i.e., the bone-cement interfacial tensile strength, of vacuum-mixed polymethylmethacrylate (PMMA) bone cement (surface roughness; Ra, 4.8 microm). Bone bonding was evaluated by a detachment test. We used unloaded cement surfaces, which could be detached from the bone. Titanium plates were developed such that a cement fill was contained within a plate that was contained within a titanium holder. Thus, a flat cement surface came into contact with traumatized bone only, and the rest of the plate had no contact with tissue. After implantation of the plate in the left tibia, 20 adult male rats were injected daily with human PTH (1-34) at 60 microg/kg per injection (n = 10) or vehicle (n = 10); the animals were killed after 4 weeks. The plates were detached from the bone by a perpendicular force. PTH treatment increased the median pull-away strength (0.21 MPa), compared with that in the vehicle-treated rats, (0.04 MPa) (P = 0.02). The results suggest that PTH treatment may have the potential to enhance the incorporation of cemented orthopedic implants.

Strong effect of PTH (1-34) on regenerating bone: a time sequence study in rats.

This study compares the effects of parathyroid hormone (PTH) treatment on new bone formation and normal baseline remodelling in rats. To study new bone formation we used a titanium bone chamber, and to study normal remodelling we used the femur and vertebrae from the same animals. One titanium bone chamber was inserted in the proximal tibia of each of 37 rats. The rats were randomly assigned to daily injections of human PTH (1-34) 60 microg/kg) or vehicle control and killed after 2, 4 or 6 weeks. The total distance of bone growth into the chamber was slightly increased by PTH. Body weight was not affected, and there was only a minor increase in trabecular density of the vertebral and femoral cancellous bone after 6 weeks. The only dramatic effect of PTH was seen in the chambers. In the controls, a marrow cavity formed in the chamber so that the cancellous density decreased from 44% to 24%, and 11% over 2, 4 and 6 weeks. In the PTH-treated animals, a dense network of bone trabeculae was found in the entire bone chamber at all times. The cancellous density increased from 48% to 60%, and 73% at 2, 4 and 6 weeks, respectively. The results suggest that PTH treatment can reduce the development of a resorption cavity. Thus, PTH in this model had a net antiresorptive effect, probably solely because it stimulated osteoblastic activity. Even though osteoclastic activity was present throughout the PTH specimens, it was not sufficient to resorb all newly formed bone. Since PTH seemed to have a greater effect on new bone formation in the chamber than on normal bone remodeling, it might become useful for improving the incorporation of orthopedic implants and stimulating fracture repair.