Improvement of the bone-screw interface strength with hydroxyapatite-coated and titanium-coated AO/ASIF cortical screws

OBJECTIVE: To evaluate whether coating AO/ASIF screws with osteoconductive materials can improve bone-screw fixation. DESIGN: Ninety-six AO/ASIF 4.5-millimeter cortical screws were divided into four paired groups and implanted in the femurs and tibiae of six sheep: Group A = standard stainless steel screws; Group B = stainless steel screws coated with highly crystalline hydroxyapatite; Group C = stainless steel screws coated with low crystalline hydroxyapatite; Group D = titanium screws coated with titanium. The screws were implanted according to the standard AO technique to an insertion torque of 2,000 Newton-millimeters. Sheep were killed at one, three, and twelve months after surgery. MAIN OUTCOME MEASURES: Extraction torque was measured on six screws from each group selected at random at time of each euthanization. Morphologic analysis of the bone-screw interface was performed on the remaining screws. RESULTS: At each euthanization the extraction torque of Group A was lower than that of the other groups (p < 0.0001). At three and twelve months the extraction torque of Group B was higher than that of Group D (p = 0.002). Morphologic results showed extensive bone-screw gap in Group A. Optimal osteointegration was observed in Groups B and C. Osteointegration of Group D was higher than that of Group A and lower than that of Groups B and C. CONCLUSIONS: It was demonstrated that AO/ASIF screws coated with osteoconductive materials achieve optimal fixation strength, even in the early phase. This fixation strength was significantly higher than that of the standard screws.     

HA-coated screws decrease the incidence of fixation failure in osteoporotic trochanteric fractures

This study was done to determine if elderly patients with trochanteric fractures and with osteoporosis could benefit from treatment with a dynamic hip screw fixed with HA-coated AO/ASIF screws. One hundred twenty patients with AO, A1, or A2 trochanteric fractures were selected. Patients were divided into two groups and randomized to receive a 135 degree-four-hole dynamic hip screw fixed with either standard lag and cortical AO/ASIF screws (Group A), or HA-coated lag and cortical AO/ASIF screws (Group B). Lag screw cutout occurred in four patients in Group A, but not in any patients in Group B. In Group A, the femoral neck shaft angle was 134 degrees +/- 5 degrees postoperatively and 127 degrees +/- 12 degrees at 6 months. In Group B, the femoral neck shaft angle was 134 degrees +/- 7 degrees postoperatively and 133 degrees +/- 7 degrees at 6 months. The Harris hip score at 6 months was 60 +/- 25 in Group A and 71 +/- 18 in Group B. The superior results of Group B can be attributed to the increased screw fixation provided by the HA-coated screws. We recommend lag screws coated with HA for dynamic hip screw fixation, especially in osteoporotic bone.     

Alendronate improves screw fixation in osteoporotic bone

BACKGROUND: Animal studies have demonstrated the efficacy of the use of bisphosphonates to enhance screw fixation in bone. In this prospective, randomized study of pertrochanteric fractures treated with external fixation, we tested whether systemic administration of bisphosphonates would improve the fixation of hydroxyapatite-coated screws implanted in osteoporotic bone. METHODS: Sixteen consecutive patients with a pertrochanteric fracture were selected. Inclusion criteria were female gender, an age of at least sixty-five years, and a bone mineral density T-score of less than -2.5 standard deviations. Exclusion criteria included bisphosphonate treatment during the two-year period prior to the fracture. Fractures were fixed with a pertrochanteric fixator and four hydroxyapatite-coated pins. Two pins were implanted in the femoral head (pin positions 1 and 2), and two were placed in the femoral diaphysis (pin positions 3 and 4). The patients were randomized either to therapy with alendronate for a three-month postoperative period (Group A) or to no therapy (Group B). The Group-A patients received an oral dose of 70 mg of alendronate per week. The fixators were removed after three months. RESULTS: All of the fractures healed, and no loss of reduction, nonunion, or delayed union was observed. The combined mean extraction torque (and standard deviation) of the pins implanted at positions 1 and 2 (cancellous bone) was 2558 +/- 1103 N/mm in Group A and 1171 +/- 480 N/mm in Group B (p < 0.0005). The combined mean extraction torque of the pins implanted at positions 3 and 4 (cortical bone) was 4327 +/- 1720 N/mm in Group A and 4075 +/- 1022 N/mm in Group B. CONCLUSIONS: These data show that weekly systemic administration of alendronate improves pin fixation in cancellous bone in elderly female patients with osteoporosis. We observed a twofold increase in extraction torque with the pins implanted in cancellous bone. These results support the use of alendronate in the treatment of osteoporotic pertrochanteric fractures to improve screw fixation in the femoral head.     

Evaluation of fixation of various screws in the lumbar vertebrae of dogs

Various screws for posterior fixation of the lumbar spine were evaluated for their firmness of fixation in mongrel dogs. Four types of screws: stainless steel, titanium base alloy, porous-coated, and hydroxyapatite-coated were inserted into vertebrae in dogs and examined for firmness of fixation in bone. Dogs were killed immediately and 2, 4, 6, and 8 weeks after insertion, and firmness of fixation was assessed by measuring the twisting force (i.e., the torque) required to loosen the screws. Also, the interface between screws and bone was histologically investigated. The hydroxyapatite-coated screws required the highest torque, compared with other screws, 2 weeks after insertion. At 8 weeks, the torque was 235% of that of the titanium alloy screws, indicating that the fixation of the hydroxyapatite-coated screws was highly stable. With the hydroxyapatite-coated screw, bone formed a direct bond with the hydroxyapatite, the connection to bone apparently being augmented with time. The results of these experiments suggest that the hydroxyapatite-coated screw has the advantage of firmer fixation in vertebrae over other conventional screws. A summary of this work was presented at the tenth annual Orthopaedic Research Meeting of the Japanese Orthopaedic Association at Karuizawa, Japan, in 1995     

Compression forces generated by Mini bone screws--a comparative study done on bone model

The compressive forces generated by the AO/ASIF 3.0 mm cannulated cancellous and 2.0 mm cortical screws, Mini-Acutrak and Herbert/Whipple small bone cannulated screws were measured in the laboratory with the use of simulated cancellous bone and a load cell washer as a means of quantifying their fixation capabilities. The Herbert/Whipple screw and the Mini-Acutrak screw were found to have nearly identical compression capabilities and provided more compression than the cortical screw. The AO/ASIF cannulated screw when used with a support screw demonstrated a compressive capacity twice that of the 2.0 mm cortical screw and higher than the headless Mini-Acutrak and Herbert/Whipple screws. The Mini-Acutrak screw produced about 70% of compression of the cancellous screw in spite of having a diameter almost half that of the cancellous screw. The Herbert/Whipple screw in spite of its larger size compared to the Mini-Acutrak produced almost the same amount of compression.     

Displaced scaphoid fractures treated with open reduction and internal fixation with a cannulated screw

BACKGROUND: This study was performed to determine if the accuracy of screw placement was improved with use of the Herbert-Whipple cannulated screw compared with use of the AO/ASIF cannulated screw and also to evaluate the functional results in patients with an acute displaced fracture of the waist of the scaphoid treated with open reduction and internal fixation with a cannulated screw. METHODS: We retrospectively reviewed the results for thirty-five patients in whom an acute displaced fracture of the waist of the scaphoid had been treated with internal fixation with use of a cannulated screw. The patients were divided into two groups; Group 1 consisted of nineteen patients managed with a 3.5-millimeter cannulated AO/ASIF screw from 1990 through 1997, and Group 2 consisted of sixteen patients managed with a Herbert-Whipple screw from 1993 through 1997. RESULTS: There were no clinical or radiographic differences between the two groups. The average time to union (and standard deviation), confirmed with tomography, was 4.2 +/- 1.2 months for Group 1 and 4.0 +/- 1.2 months for Group 2. Both screws significantly improved the alignment of the scaphoid and decreased carpal collapse (p < 0.01). Importantly, the use of either cannulated screw improved the height-to-length ratio and the lateral intrascaphoid angle, which were correlated with an increase in the range of motion of the wrist (r = 0.584 and 0.625). In addition, both screws allowed for accurate placement in the central portion of the proximal pole. Regardless of the type of screw used, the time to union increased with increasing age of the patient (r = 0.665) and with increasing initial displacement of the fracture (r = 0.541). Within both groups, the time to union was longer for the patients who smoked (p < 0.01). CONCLUSIONS: Within both groups, cannulated screw fixation maintained the corrected fracture alignment and promoted healing and return of function. Our study shows cannulated screws to be a safe and effective method of treatment.     

The effect of surface material and roughness on bone screw stability.

OBJECTIVE: To evaluate the effects of osteointegration ability and surface texture on bone screw interface stability in three different groups of titanium screws. DESIGN: Sixty titanium tapered screws were used: twenty screws were polished, uncoated (Group A); twenty screws were rough, coated with titanium (Group B); and twenty screws were rough, coated with fluor-hydroxyapatite (Group C). Thirty screws, ten per group, were implanted in the femurs and tibiae of two sheep, which were euthanized one month after surgery. The remaining thirty screws, ten per group, were implanted in the femurs and tibiae of another two sheep, which were euthanized three months after surgery. RESULTS: At one month, extraction torque of Group C was higher than that of Group A (p = 0.042). At three months, extraction torque of Group C was higher than that of Group A (p < 0.0001) and Group B (p < 0.0001). At three months, extraction torque of Group C was higher compared with the corresponding insertion torque (p < 0.0001) and compared with the corresponding extraction torque at one month (p < 0.0001). At one and three months, a high percentage of bone-screw contact was observed histologically in Groups B and C. A continuous gap with fibrous tissue encapsulation was observed in Group A. CONCLUSIONS: This study shows that the osteointegration ability provided by the type of coating is a very important parameter for optimizing the bone-screw stability. Surface texture is also important. By using screws with optimal osteointegration ability, very positive clinical consequences can be expected.     

Effect of insertion torque on bone screw pullout strength

The effect of insertion torque on the holding strength of 4.5-mm ASIF/AO cortical bone screws was studied in vitro. Screw holding strength was determined using an Instron materials testing machine (Bristol, United Kingdom) on 55 lamb femora and 30 human tibiocortical bone sections. Holding strength was defined as tensile stress at pullout with rapid loading to construct failure. Different insertion torques were tested, normalizing to the thickness of cortical bone specimen engaged. These represented low, intermediate, high, and thread-damaging insertion torque. All screws inserted with thread-damaging torque and single cortex engaging screws inserted to high torque tightening moments showed diminished holding strength. This loss of strength amounted to 40%-50% less than screws inserted with less torque.     

Bioabsorbable expansion bolt fixation in anterior cruciate ligament reconstruction

The current study evaluated initial fixation strength of a bioabsorbable expansion bolt compared with interference screw fixation in anterior cruciate ligament reconstruction using a bone-patellar tendon-bone graft. Thirty calf tibial plateaus with adjacent patella and extensor ligaments were used. Bioabsorbable poly-L-lactide interference screws were used for graft fixation in Group I, titanium screws in Group II, and bioabsorbable poly-DL-lactide expansion bolts were used in Group III. The mean force-to-failure (+/- standard deviation) in the three groups was 487 +/- 205 N, 713 +/- 218 N, and 594 +/- 224 N, respectively. The differences between Groups I and II were significant. No statistical differences were found regarding stiffness. Graft damage was significantly less in Group III compared with screw fixation. The fixation concept of an expansion bolt shows similar fixation strength and less graft damage compared with the established interference screw fixation. Because of the total absence of torque forces in contrast to bioabsorbable screws, the risk of implant breakage is minimized.     

The significance of radiolucent zones surrounding pedicle screws. Definition of screw loosening in spinal instrumentation

We examined the radiographs from a prospective clinical study of fixation by pedicle screws and those from an experimental study in a sheep model. In the clinical study, instruments were removed from 21 patients after implantation for 11 to 16 months and the extraction torques of the screws were recorded. A structured protocol was used for the radiological examinations. In the experimental study, loaded pedicle screw instrumentations were implanted in the sheep for six or 12 weeks. After radiological examination the pull-out resistance and the histological characteristics were studied. In the clinical study, all screws with radiolucent zones had a significantly reduced mean extraction torque compared with screws without radiolucent zones (16 +/- 10 Ncm v 403 +/- 220 Ncm; p < 0.0001). In the experimental study the mean maximum pull-out resistance for the screws with radiolucent zones was significantly lower than for those with no radiolucency (243 +/- 156 N v2214 +/- 578 N; p = 0.0006) and the mean bone-to-screw contact was reduced for screws with zones compared with those without zones (8 +/- 9% v 55 +/- 29%; p = 0.0002). Our findings showed that all screws with radiolucent zones had low extraction torques or low pull-out resistance. A radiolucent zone is a good indicator of loosening of a pedicle screw.     

Stripping torque as a predictor of successful internal fracture fixation

BACKGROUND: Internal fixation of fractures using plates and screws is a common method of treatment. Occasionally the internal fixation fails prior to fracture healing. This often requires revision surgery. Determining the force that internal fixation needs to withstand postoperatively would enable this force to be applied intraoperatively as a test to predict successful fixation. The purpose of the present paper was to determine the minimum stripping torque needed to predict successful internal fixation strength. METHODS: The pull-out strength and stripping torque relationships of 4.5-mm cortical bone screws in Sawbones polyurethane foam were determined. Screw forces were directly measured using an LCM load cell washer on a model intertrochanteric neck of femur fracture fixed with 135 degrees 4-hole pin and plate loaded to single leg stance conditions. Additionally a 135 degrees 4-hole pin and plate was mounted on foam blocks and loaded until failure of the shaft screws from the foam occurred. Predicted stripping torque/yield load was determined. RESULTS: Pull-out strength and stripping torque of 4.5-mm cortical bone screws in polyurethane foam have a high degree of linear correlation R(2) = 0.95. Direct measurement of shaft screw forces at single leg stance conditions were 585-686 N. This correlated with a stripping torque of 0.9 Nm. Load to yield testing at single leg stance conditions corresponded to a stripping torque of 1.8 Nm. CONCLUSION: Withstanding 0.9-1.8 Nm of torque during insertion of the femoral shaft screws of a 135 degrees 4-hole pin and plate predicts that the construct will successfully withstand single leg stance.     

Improved extraction torque of hydroxyapatite-coated pedicle screws

Loosening of the screws is a problem in instrumentation with pedicle screws. Coating with hydroxyapatite improves the holding characteristics for metal implants, but the possible effects on the anchorage of pedicle screws have not been described. In this study, seven patients were operated on with spinal instrumentation using four stainless steel pedicle screws. Hydroxyapatite-coated screws were used in either the upper or the lower of the instrumented levels. The insertion torque was measured. In four cases the screws were removed after 10–22 months and the extraction torque was measured. The mean insertion torque was found to be significantly greater in the hydroxyapatite-coated screws (107 Ncm) than in the standard screws (76 Ncm). In three cases, the extraction torque for the hydroxyapatite-coated screws exceeded the range for the torque wrench (600 Ncm), while the conventional screws were loose (< 5 Ncm). In one case, the extraction torque was 475 and 550 Ncm for the coated screws, and 5 and 25 Ncm for the conventional screws. The difference in extraction torque was significant. Hydroxyapatite coating was shown to have improved the purchase of pedicle screws very effectively. By using fully coated screws, as in the present study, extraction was extremely difficult compared to extraction of conventional stainless steel screws, which were regularly loose. By reducing the area of the screws that is coated, it may be possible to achieve an enhanced purchase while extraction will be easier when compared to fully coated screws. Received: 11 October 1999 Revised: 7 February 2000 Accepted: 8 May 2000     

Transplantatfixation bei der vorderen Kreuzbandrekonstruktion Metall- vs. bioresorbierbare Polyglykonatinterferenzschraube – Eine prospektive randomisierte Studie von 40 Patienten

To overcome some of the potential problems (e.g. hardware removal during revision surgery) of metal interference screws used for patellar tendon anterior cruciate ligament reconstruction, bioabsorbable screws have recently been introduced. Forty patients who underwent endoscopic ACL reconstruction using patella tendon autograft were included in the study, they were randomized intraoperatively to either Group A (femoral bone block fixation: polyglyconate screw; tibial: metal screw) or Group B (both bone blocks fixed with metal interference screws). The patients were evaluated clinically preoperatively as well as 6 weeks, 3 months 12 months and 24 months post op. Lysholm Score at 24 months was 98.1 +/- 2.3 for Group A and 97.7 +/- 3.0 for Group B. Tegner Score was 7.4 +/- 1.1 for Group A and 7.5 +/- 0.8 for Group B. Two years post op overall IKDC-Score for group A was 5.6% normal, 88.8% nearly normal and 5.6% abnormal. The result for group B was 11.1%, 77.8% and 11.1%, respectively. KT-1000 (at 89 N) at two years revealed a side to side difference of 1.5 +/- 0.3 mm (Group A) and 1.6 +/- 0.7 (Group B). The results of the two groups did not show significant differences at any stage of follow up. In our study polyglyconate interference screw fixation for patellar tendon grafts has not found to be associated with increased clinical complications. It provided equivalent fixation and clinical results compared to metal screws.     

Die messung von anzugsdrehmoment und vorspannkraft bei schenkelhalsverschraubung im experiment

Zusammenfassung An proximalen Leichenfemora über Sechzigjähriger wurde der Schenkelhals osteotomiert und eine Osteosynthese mit vier AO-Spongiosaschrauben 6.5 mm durchgeführt. An diesen wurde das Anzugsdrehmoment bestimmt and es wurden die Vorspannkräfte mittels Dehnungsmeßstreifen ermittelt. Die Vorspannkräfte liegen im Mittel zwischen 1000 (Gewinde 16 mm) and 2000 N (Gewinde 32 mm). Der von vier Spongiosaschrauben anfangs aufgebrachte interfragmentäre Druck am Schenkelhals kann mit durchschnittlich 4000 bis 800O N angegeben werden.

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The experimental measurement of torque and axial force of 6.5 mm ASIF lag screws in fixation of femoral neck fractures

Summary An experimental study was undertaken to evaluate the torque and axial force of the 6.5 mm ASIF lag screw implanted in the femoral neck after a subcapital osteotomy. The experiments were carried out on autopsied bones of more than 60 years of age. The average torque was 204 cmN, the screws in the upper half of the femoral head showing a higher torque than those in the lower half. The axial tension was measured by strain gauge transducer. Depending on the length of the thread of the screw (16 or 32 mm) the average tension reaches 1162 N or 1960 N.By tightening the screw until the impression of optimum fit is obtained, 70% of the maximum axial force is reached. It can therefore be estimated that at the beginning an interfragmentary pressure of 4000 to 8000 N is exerted on a medial femoral neck fracture fixed by four ASIF lag screws.

     

The effects of cyclic loading on pull-out strength of sacral screw fixation: an in vitro biomechanical study

STUDY DESIGN: The pull-out strength of sacral screw fixation after cyclic loading was tested using young human cadaveric specimens. OBJECTIVES: To evaluate the effects of fatigue loading on the pull-out strength of medial and lateral unicortical and bicortical sacral screws and to correlate the pull-out strength with sacral bone density and the screw insertion torque. SUMMARY OF BACKGROUND DATA: The immediate biomechanical effects of depth of penetration, screw orientation, and bone density on sacral screw fixation have been studied in aged cadaveric specimens. The effect of cyclic loading on the pull-out strength of sacral screw fixation is unknown, however, and data from young specimens is rare. METHODS: Eleven fresh specimens of human sacrum were used in this study. Bone mineral density at the vertebral body and the ala were determined by peripheral quantitative computed tomography. Seven-millimeter compact Cotrel-Dubousset sacral screws were inserted into the sacrum anteromedially and anterolaterally, both unicortically and bicortically, and the insertion torque for each screw was measured. Cyclic loading from 40 to 400 N was applied to each screw at a frequency of 2 Hz up to 20,000 cycles. Pull-out tests were conducted after completion of the fatigue tests. RESULTS: The average bone density was 0.38 +/- 0.08 g/mL at the S1 body and 0.24 +/- 0.05 g/mL at the S1 ala. The insertion torque and average pull-out force after cyclic loading were significantly higher for bicortical fixation than for unicortical fixation for a particular screw alignment. The pull-out strength and insertion torque of medially oriented fixation was always higher than that for lateral fixation, however, regardless of whether the insertion was unicortical or bicortical. The pull-out force of unicortical and bicortical medial screw fixations after cyclic loading showed significant linear correlations with both the insertion torque and the bone mineral density of the S1 body. CONCLUSIONS: In a young population, screw orientation (anterolateral or anteromedial) was more important in determining pull-out strength than screw depth (unicortical or bicortical) after fatigue loading, anteromedially directed screws being significantly stronger than laterallyplaced screws. Bone mineral density of the S1 body andinsertion torque were good preoperative and intraoperative indicators of screw pull-out strength.     

Pedicle screw fixation strength: pullout versus insertional torque.

BACKGROUND CONTEXT: Researchers studying early pedicle screw designs have suggested that pullout strength and insertional torque are correlated. For surgeons using pedicle screws, insertional torque is widely believed to be a good predictor of pullout strength and initial stability of the screw and construct. How appropriate is this assumption when applied to new screw and thread designs? PURPOSE: This study investigated the correlation between insertional torque and pullout strength of three different pedicle screw designs, with different insertional torque characteristics. We hypothesized that a significant increase in insertional torque would indicate a commensurate increase in pullout strength. STUDY DESIGN: Biomechanical analysis of instrumented vertebral specimens. METHODS: Calf lumbar vertebra were prepared and instrumented with one of three pedicle screws. Pilot hole preparation was standardized and coaxial orientation was confirmed by direct inspection. Screws did not penetrate the pedicle cortex or abut or penetrate the anterior vertebral cortex. Any specimen with pedicle wall breach was discarded. The pedicles were instrumented with one of three screws: 1) 7.5 x 40 mm conical, asymmetric progressive thread (Xia; Stryker Spine, Allendale, NJ), 2) 7.5 x 40 mm conical with traditional V-shaped thread (Osteonics, Stryker Spine, Allendale, NJ)) or 3) 6.5 x 40 mm cylindrical with V thread (Osteonics, cylindrical). Paired testing allowed individual screws to be directly compared with a contralateral "control." Insertional torque and peak torque values were recorded for each rotation up to full insertion. Pullout testing was conducted at a rate of 1 mm/minute. Load-displacement data were recorded at 20 Hz. Stiffness was considered the slope of the most linear part of the curve before the yield point. RESULTS: Peak loads for 7.5 conical Xia screws measured 1,783+/-589.1 N compared with 1,943.0+/-625.8 N for 7.5 conical Osteonics screws and 1,641.0+/-356.7 N for 6.5 cylindrical Osteonics screws. The peak insertional torque values were 6.7+/-1.9 Nm (158% greater than control), 4.5+/-1.1 Nm (73% greater than control) and 2.6+/-0.7 Nm, respectively. Insertional torques for Xia screws were significantly greater than conical (p=.001) and cylindrical Osteonics screws (p<.0001), and insertional torques for Osteonics conical screws were significantly greater than those of cylindrical screws (p<.0001). Although pullout loads for the conical Osteonics screws were consistently higher than either the Xia or Osteonics cylindrical screws, the differences were not significant (p>.05). There was no significant correlation between pullout strength and insertional torque (p>.05). CONCLUSIONS: This unexpected result is best explained by the progressively narrowing flutes of the Xia screw, which compact the trabeculae into a smaller volume as the screw nears full insertion. The trapezoidal threads also increase contact with the cortical surface area and compress trabeculae toward the cortex, thus creating greater friction and higher torque values. This increase in torque did not translate into a commensurate increase in pullout strength, where trabeculae fail in shear.     

The use of cancellous screws as an alternative in plate fixation of fibular fractures

Altering standard AO/ASIF techniques when performing open reduction with internal fixation of fibular fractures by using cancellous screws in place of cortical screws appears to be a viable option. By eliminating a few steps in screw application, the additive effect can lead to a significant decrease in operative and tourniquet time. Close inspection of hardware postoperatively has failed to show any signs of screw loosening or delay in fracture healing. In those cases when hardware has had to be removed, it was noted that screw tightness had not been altered. This topic has been anecdotally discussed within our institution and will be the subject of future scientific investigation. The authors feel that the application of fibular plates with cancellous screws is an acceptable alternative to standard orthopedic technique. Although we do not routinely vary from standard AO/ASIF technique, we feel this particular alteration does not affect outcome other than reducing surgical and tourniquet time, therefore reducing the surgical risks.     

The fixation strength of hydroxyapatite-coated Schanz screws and standard stainless steel Schanz screws in lower extremity lengthening

Introduction: Lengthening procedures are often complicated by loosening of pins. It has been reported that coating with hydroxyapatite improves fixation and reduces the rate of pin-track infection. Materials and Methods: We compared 47 hydroxyapatite-coated Schanz screws (HA screws) in 12 monolateral fixators mounted at the University Hospital Hamburg–Eppendorf with 45 standard stainless steel screws in 9 monolateral fixators mounted at the St Josefs-Hospital Wiesbaden by measuring the insertion and extraction torque values. The average implantation period was 7 months for the hydroxyapatite-coated screws and 5.4 months for the uncoated screws. We established the quotient of the maximum extraction torque over insertion torque which shows the change in the fixation strength with respect to time, the fixation index. It eliminates the influence of the varying pin–bone contact. Results: There was no significant difference in the rate of infection. In the Schanz screws without signs of infection the index was 1.92 for the HA screws and 0.76 for the stainless steel screws (P = 0.0002) giving evidence of the improvement of the fixation by the coating. Conclusion: HA coating resulted in improved fixation of Schanz screws in bone and may be useful in prolonged external fixation of the lower leg. The fixation index proved to be a simple tool for the evaluation of the fixation strength of Schanz screws.     

Distal tibiofibular syndesmosis fixation: A cadaveric, simulated fracture stabilization study comparing bioabsorbable and metallic single screw fixation

Metal screws that are used for ruptured tibiofibular syndesmosis repair are often removed within 3 months of placement, suggesting the utility of bioabsorbable screws. A biomechanical study was performed to compare fixation of a simulated syndesmosis separation with a 5-mm oriented copolymer bioabsorbable (82:18 poly-L-lactic acid/poly-glycolic acid) versus a stainless steel screw. Eight pairs of cadaveric lower-leg specimens were cleaned and a pronation external rotation-type injury was created in each. The syndesmosis was fixed with a single, tricortical bioabsorbable screw in 1 ankle and a metal screw in the contralateral ankle (matched pairs). Sequential testing of the specimens showed that torsional stiffness of the fixed, relative to intact, specimens was nearly equivalent (0.730 +/- 0.260 for copolymer, 0.770 +/- 0.300 for stainless steel; P = .401). Application of 1000 cycles of axial load (90 to 900 N) resulted in a significant decrease ( P < .0001) in axial stiffness for each fixation method, but the relative decrease was equivalent for both ( P = .211). Failure torque (17.8 +/- 8.3 N.m copolymer, 21.0 +/- 11.5 N.m stainless steel; P = .238) and angle of rotation at failure (47.9 +/- 13.6 degrees copolymer, 42.0 +/- 11.5 degrees stainless steel; P = .199) were also nearly equivalent. It appears that the 5.0-mm diameter copolymer screw is biomechanically equivalent to the 5.0-mm diameter stainless steel screw for repair of syndesmosis disruption.     

In vivo effects of coating loaded and unloaded Ti implants with collagen, chondroitin sulfate, and hydroxyapatite in the sheep tibia.

The in vivo effects of coating titanium implants with organic extracellular matrix molecules were examined in the sheep tibia. Titanium screws (5.0 mm) were coated with type I collagen (Ti/Coll) or type I collagen and chondroitin sulfate (Ti/Coll/CS) by biomimetic fibrillogenesis. Uncoated screws (Ti) and screws coated with hydroxyapatite (Ti/HA) served as control. Six adult female sheep received one screw of each type to stabilize a midshaft tibial fracture with external fixation. Four cylindrical implants of 4-mm outer diameter and 3.3-mm inner diameter with the same coatings were inserted into the tibial head. No pin track infections were seen at the time of implant retrieval 6 weeks after implantation. Extraction torque was greater for Ti/HA (1181 Nmm) and Ti/Coll/CS (1088 Nmm) compared to Ti/Coll (900 Nmm) and Ti (904 Nmm) [N.S.]. Newly formed bone was noted around all coated screws within the medullary cavity. Macrophage and osteoclast activity was significantly reduced around Ti/Coll/CS in both types of implants compared to uncoated controls (p < 0.05). Osteoblast activity was significantly increased around loaded Ti/Coll and Ti/Coll/CS screws compared to uncoated Ti screws (p < 0.05). Microtomographic evaluation (SRmicroCT) revealed no significant differences in new bone formation around the unloaded tibial head implants.Coating of external fixation devices with of type I collagen and chondroitin sulfate appears to have similar effects with respect to stability and bone healing as HA but with less osteoclast activity. These findings were more pronounced under loaded than unloaded conditions in the sheeptibia.