Predictors of femoral neck fracture following hip resurfacing: a cadaveric study.

We aimed to establish if radiological parameters, dual energy x-ray absorptiometry (DEXA) and quantitative CT (qCT) could predict the risk of sustaining a femoral neck fracture following hip resurfacing. Twenty-one unilateral fresh frozen femurs were used. Each femur had a plain digital anteroposterior radiograph, DEXA scan and qCT scan. Femurs were then prepared for a Birmingham Hip Resurfacing femoral component and loaded to failure. Results demonstrated that gender and qCT measurements showed strong correlation with failure load. QCT could be used as an individual measure to predict risk of post-operative femoral neck fracture. However, when qCT is unavailable; gender, pre-operative DEXA scan and Neck Width measurements can be used together to assess risk of post-operative femoral neck fracture in patients due to undergo hip resurfacing.

Changes in femoral cortical porosity after reaming and intramedullary canal preparation in a canine model.

This study examined changes in femoral cortical porosity resulting from femoral canal preparation during cemented total hip arthroplasty (THA). Twenty-four canines were randomly assigned to 3 groups: (1) reaming only, (2) cementing without pressurization, and (3) cementing with pressurization. Femoral cortical porosity was measured from histologic samples of the femurs at 7 positions. Reaming during canal preparation significantly increased cortical porosity. Cementing further increased cortical porosity, whereas pressurization of cement helped to counteract the increase in cortical porosity caused by cementing alone. Cortical porosity was considered to be a marker for bone mineral density (BMD) during the early phase of peri-implant healing around cemented stems. To maximize bone mineral density after cemented total hip arthroplasty, we suggest using implants that do not require reaming and pressurizing cement appropriately.

The biomechanics of three different fracture fixation implants for distal femur repair in the presence of a tumor-like defect.

The femur is the most common long bone involved in metastatic disease. There is consensus about treating diaphyseal and epiphyseal metastatic lesions. However, the choice of device for optimal fixation for distal femur metaphyseal metastatic lesion remains unclear. This study compared the mechanical stiffness and strength of three different fixation methods. In 15 synthetic femurs, a spherical tumor-like defect was created in the lateral metaphyseal region, occupying 50% of the circumference of the bone. The defect was filled with bone cement and fixed with one of three methods: Group 1 (retrograde nail), Group 2 (lateral locking plate), and Group 3 (lateral nonlocking periarticular plate). Constructs were tested for mechanical stiffness and strength. There were no differences between groups for axial stiffness (Group 1, 1280 +/- 112 N/mm; Group 2, 1422 +/- 117 N/mm; and Group 3, 1403 +/- 122N/mm; p = 0.157) and offset torsional strength (Group 1, 1696 +/- 628N; Group 2, 1771 +/- 290N; and Group 3, 1599 +/- 253 N; p = 0.816). In the coronal plane, Group 2 (296 +/- 17 N/mm) had a higher stiffness than Group 1 (263 +/- 17N/mm; p = 0.018). In the sagittal plane, Group 1 (315 +/- 9 N/mm) had a higher stiffness than Group 3 (285 +/- 19 N/mm; p = 0.028). For offset torsional stiffness, Group 1 (256 +/- 23 N/mm) had a higher value than Group 3 (218 +/- 16 N/mm; p = 0.038). Group 1 had equivalent performance to both plating groups in two test modes, and it was superior to Group 3 in two other test modes. Since a retrograde nail (i.e. Group 1) would require less soft-tissue stripping in a clinical context, it may be the optimal choice for tumor-like defects in the distal femur.

The biomechanical effect of torsion on humeral shaft repair techniques for completed pathological fractures.

In the presence of a tumor defect, completed humeral shaft fractures continue to be a major surgical challenge since there is no "gold standard" treatment. This is due, in part, to the fact that only one prior biomechanical study exists on the matter, but which only compared 2 repair methods. The current authors measured the humeral torsional performance of 5 fixation constructs for completed pathological fractures. In 40 artificial humeri, a 2-cm hemi-cylindrical cortical defect with a transverse fracture was created in the lateral cortex. Specimens were divided into 5 different constructs and tested in torsion. Construct A was a broad 10-hole 4.5-mm dynamic compression plate (DCP). Construct B was the same as A except that the screw holes and the tumor defect were filled with bone cement and the screws were inserted into soft cement. Construct C was the same as A except that the canal and tumor defect were filled with bone cement and the screws were inserted into dry cement. Construct D was a locked intramedullary nail inserted in the antegrade direction. Construct E was the same as D except that bone cement filled the defect. For torsional stiffness, construct C (4.45 ± 0.20 Nm/deg) was not different than B or E (p > 0.16), but was higher than A and D (p < 0.001). For failure torque, construct C achieved a higher failure torque (69.65 ± 5.35 Nm) than other groups (p < 0.001). For the failure angle, there were no differences between plating constructs A to C (p ≥ 0.11), except for B versus C (p < 0.05), or between nailing groups D versus E (p = 0.97), however, all plating groups had smaller failure angles than both nailing groups (p < 0.05). For failure energy, construct C (17.97 ± 3.59 J) had a higher value than other groups (p < 0.005), except for A (p = 0.057). Torsional failure always occurred in the bone in the classic "spiral" pattern. Construct C provided the highest torsional stability for a completed pathological humeral shaft fracture.

Biomechanical measurements of surgical drilling force and torque in human versus artificial femurs.

Few experimental studies have examined surgical drilling in human bone, and no studies have inquired into this aspect for a popular commercially-available artificial bone used in biomechanical studies. Sixteen fresh-frozen human femurs and five artificial femurs were obtained. Cortical specimens were mounted into a clamping system equipped with a thrust force and torque transducer. Using a CNC machine, unicortical holes were drilled in each specimen at 1000 rpm, 1250 rpm, and 1500 rpm with a 3.2 mm diameter surgical drill bit. Feed rate was 120 mm/min. Statistical significance was set at p < 0.05. Force at increasing spindle speed (1000 rpm, 1250 rpm, and 1500 rpm), respectively, showed a range for human femurs (198.4 ± 14.2 N, 180.6 ± 14.0 N, and 176.3 ± 11.2 N) and artificial femurs (87.2 ± 19.3 N, 82.2 ± 11.2 N, and 75.7 ± 8.8 N). For human femurs, force at 1000 rpm was greater than at other speeds (p ≤ 0.018). For artificial femurs, there was no speed effect on force (p ≥ 0.991). Torque at increasing spindle speed (1000 rpm, 1250 rpm, and 1500 rpm), respectively, showed a range for human femurs (186.3 ± 16.9 N·mm, 157.8 ± 16.1 N·mm, and 140.2 ± 16.4 N·mm) and artificial femurs (67.2 ± 8.4 N·mm, 61.0 ± 2.9 N·mm, and 53.3 ± 2.9 N·mm). For human femurs, torque at 1000 rpm was greater than at other speeds (p < 0.001). For artificial femurs, there was no difference in torque for 1000 rpm versus higher speeds (p ≥ 0.228), and there was only a borderline difference between the higher speeds (p = 0.046). Concerning human versus artificial femurs, their behavior was different at every speed (force, p ≤ 0.001; torque, p < 0.001). For human specimens at 1500 rpm, force and torque were linearly correlated with standardized bone mineral density (sBMD) and the T-score used to clinically categorize bone quality (R ≥ 0.56), but there was poor correlation with age at all speeds (R ≤ 0.37). These artificial bones fail to replicate force and torque in human cortical bone during surgical drilling. To date, this is the largest series of human long bones biomechanically tested for surgical drilling.

A preliminary biomechanical study of cyclic preconditioning effects on canine cadaveric whole femurs.

Biomechanical preconditioning of biological specimens by cyclic loading is routinely done presumably to stabilize properties prior to the main phase of a study. However, no prior studies have actually measured these effects for whole bone of any kind. The aim of this study, therefore, was to quantify these effects for whole bones. Fourteen matched pairs of fresh-frozen intact cadaveric canine femurs were sinusoidally loaded in 4-point bending from 50 N to 300 N at 1 Hz for 25 cycles. All femurs were tested in both anteroposterior (AP) and mediolateral (ML) bending planes. Bending stiffness (i.e., slope of the force-vs-displacement curve) and linearity R(2) (i.e., coefficient of determination) of each loading cycle were measured and compared statistically to determine the effect of limb side, cycle number, and bending plane. Stiffnesses rose from 809.7 to 867.7 N/mm (AP, left), 847.3 to 915.6 N/mm (AP, right), 829.2 to 892.5 N/mm (AP, combined), 538.7 to 580.4 N/mm (ML, left), 568.9 to 613.8 N/mm (ML, right), and 553.8 to 597.1 N/mm (ML, combined). Linearity R(2) rose from 0.96 to 0.99 (AP, left), 0.97 to 0.99 (AP, right), 0.96 to 0.99 (AP, combined), 0.95 to 0.98 (ML, left), 0.94 to 0.98 (ML, right), and 0.95 to 0.98 (ML, combined). Stiffness and linearity R(2) versus cycle number were well-described by exponential curves whose values leveled off, respectively, starting at 12 and 5 cycles. For stiffness, there were no statistical differences for left versus right femurs (p = 0.166), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p < 0.0001). Similarly, for linearity, no statistical differences were noted due to limb side (p = 0.533), but there were effects due to cycle number (p < 0.0001) and AP versus ML bending plane (p = 0.006). A minimum of 12 preconditioning cycles was needed to fully stabilize both the stiffness and linearity of the canine femurs. This is the first study to measure the effects of mechanical preconditioning on whole bones, having some practical implications on research practices.

Biomechanical measurements of axial crush injury to the distal condyles of human and synthetic femurs.

Few studies have evaluated the 'bulk' mechanical properties of human longbones and even fewer have compared human tissue to the synthetic longbones increasingly being used by researchers. Distal femur fractures, for example, comprise about 6% of all femur fractures, but the mechanical properties of the distal condyles of intact human and synthetic femurs have not been well quantified in the literature. To this end, the distal portions of a series of 16 human fresh-frozen femurs and six synthetic femurs were prepared identically for mechanical testing. Using a flat metal plate, an axial 'crush' force was applied in-line with the long axis of the femurs. The two femur groups were statistically compared and values correlated to age, size, and bone quality. Results yielded the following: crush stiffness (human, 1545 +/- 728 N/mm; synthetic, 3063 +/- 1243 N/mm; p = 0.002); crush strength (human, 10.3 +/- 3.1 kN; synthetic, 12.9 < or = 1.7 kN; p = 0.074); crush displacement (human, 6.1 +/- 1.8 mm; synthetic, 2.8 +/- 0.3 mm; p = 0.000); and crush energy (human, 34.8 +/- 15.9J; synthetic, 18.1 +/- 5.7J; p = 0.023). For the human femurs, there were poor correlations between mechanical properties versus age, size, and bone quality (R2 < or = 0.18), with the exception of crush strength versus bone mineral density (R2 = 0.33) and T-score (R2 = 0.25). Human femurs failed mostly by condyle 'roll back' buckling (15 of 16 cases) and/or unicondylar or bicondylar fracture (7 of 16 cases), while synthetic femurs all failed by wedging apart of the condyles resulting in either fully or partially displaced condylar fractures (6 of 6 cases). These findings have practical implications on the use of a flat plate load applicator to reproduce real-life clinical failure modes of human femurs and the appropriate use of synthetic femurs. To the authors' knowledge, this is the first study to have done such an assessment on human and synthetic femurs.

Biomechanical measurements of cortical screw stripping torque in human versus artificial femurs.

Femur fracture plates are applied using cortical bone screws. Surgeons do this manually by subjective 'feel' without monitoring torque. Few studies have quantified stripping torque in human bone. No studies have measured stripping torque in the artificial bones from Sawbones (Vashon, WA, USA) that are frequently used in biomechanical studies. The present aim was to measure stripping torque of cortical screws in human versus artificial femurs. Sixteen fresh-frozen human femurs and eight artificial femurs were used. Using a digital torque screwdriver, each femur had a 3.5-mm diameter uni-cortical screw manually inserted into the anterior midshaft until failure of the screw-bone interface. Results were normalized by cortical thickness and the screw-bone interfacial area. There were no statistical differences in human versus artificial data, respectively, for stripping torque (1741 +/- 442 N.mm, 2012 +/- 176 N.mm, p = 0.11), stripping torque/thickness (313 +/- 59 N, 305 +/- 30 N, p = 0.74), and stripping torque area (28.5 +/- 5.3 N/mm, 27.8 +/- 2.8 N/mm, p = 0.74). Artificial unicortical thickness (6.6 + 0.3 mm) was greater than human thickness (5.6 +/- 1.1 mm) (p = 0.02). For human specimens, there was a moderate linear correlation of absolute and normalized stripping torque versus standardized bone mineral density (R > or = 0.32) and clinical T-score (R = 0.29), but not with age (R < or = 0.29). Surgeons should be aware of the stripping torque limits for human femurs and potentially take steps to monitor these values during surgery. The artificial femurs being increasingly used in research accurately replicate human cortical properties during screw insertion. To date, this is the first series of human femurs evaluated for cortical screw stripping.

The effect of intramedullary reaming on a diaphyseal bone defect of the tibia.

BACKGROUND: The effect of intramedullary reaming on diaphyseal tibial defects has not been examined in the literature. The present aim was to relate the extent of reaming to angiogenesis and bone formation occurring around a critical-sized defect in the tibia for two scenarios, namely, when the bone defect is left empty and when the bone defect is treated with autograft. METHODS: Eleven canines were allocated into two groups, namely, empty (n=5) or iliac crest autograft (n=6). All tibiae were reamed to 7.0 mm and fixed with a 6.5-mm statically locked intramedullary nail after creation of an 8.0-mm diaphyseal defect. The extent of reaming of the canal was dependent on the cross-sectional area of the tibia, because all tibiae were reamed to 7.0 mm. Fluorescent markers were administered at different times: calcein green (6 weeks), xylenol orange (9 weeks), and tetracycline (11 weeks and 14 weeks). Animals were sacrificed at 15 weeks and perfused with a barium compound. Analysis consisted of radiography, micro-computed tomography scan, and histology. RESULTS: Linear regression analysis of percent bone volume and canal area provided a Pearson correlation coefficient of r=0.925 (p=0.025) for empty samples and r=0.244 (p=0.641) for autograft samples. Linear regression analysis of percent vasculature volume and canal area provided a Pearson correlation coefficient of r=0.784 (p=0.117) for empty samples and r=-0.146 (p=0.783) for autograft samples. Bone formation rates were reported as the distance between the fluorescent labels and were less within the endosteum, cortex, and periosteum, with extensive reaming in empty samples. CONCLUSIONS: The results suggest that limited reaming may be beneficial to the acute management of tibial shaft fractures with a bone defect.

A preliminary biomechanical assessment of a polymer composite hip implant using an infrared thermography technique validated by strain gage measurements.

With the resurgence of composite materials in orthopaedic applications, a rigorous assessment of stress is needed to predict any failure of bone-implant systems. For current biomechanics research, strain gage measurements are employed to experimentally validate finite element models, which then characterize stress in the bone and implant. Our preliminary study experimentally validates a relatively new nondestructive testing technique for orthopaedic implants. Lock-in infrared (IR) thermography validated with strain gage measurements was used to investigate the stress and strain patterns in a novel composite hip implant made of carbon fiber reinforced polyamide 12 (CF/PA12). The hip implant was instrumented with strain gages and mechanically tested using average axial cyclic forces of 840 N, 1500 N, and 2100 N with the implant at an adduction angle of 15 deg to simulate the single-legged stance phase of walking gait. Three-dimensional surface stress maps were also obtained using an IR thermography camera. Results showed almost perfect agreement of IR thermography versus strain gage data with a Pearson correlation of R(2) = 0.96 and a slope = 1.01 for the line of best fit. IR thermography detected hip implant peak stresses on the inferior-medial side just distal to the neck region of 31.14 MPa (at 840 N), 72.16 MPa (at 1500 N), and 119.86 MPa (at 2100 N). There was strong correlation between IR thermography-measured stresses and force application level at key locations on the implant along the medial (R(2) = 0.99) and lateral (R(2) = 0.83 to 0.99) surface, as well as at the peak stress point (R(2) = 0.81 to 0.97). This is the first study to experimentally validate and demonstrate the use of lock-in IR thermography to obtain three-dimensional stress fields of an orthopaedic device manufactured from a composite material.

Biomechanical measurements of torsion-tension coupling in human cadaveric femurs.

The mechanical behavior of human femurs has been described in the literature with regard to torsion and tension but only as independent measurements. However, in this study, human femurs were subjected to torsion to determine if a simultaneous axial tensile load was generated. Fresh frozen human femurs (n=25) were harvested and stripped of soft tissue. Each femur was mounted rigidly in a specially designed test jig and remained at a fixed axial length during all experiments. Femurs were subjected to external and internal rotation applied at a constant angulation rate of 0.1 deg/s to a maximum torque of 12 N m. Applied torque and generated axial tension were monitored simultaneously. Outcome measurements were extracted from torsion-versus-tension graphs. There was a strong relationship between applied torsion and the resulting tension for external rotation tests (torsion/tension ratio=551.7±283.8 mm, R(2)=0.83±0.20, n=25), internal rotation tests (torsion/tension ratio=495.3±233.1 mm, R(2)=0.87±0.17, n=24), left femurs (torsion/tension ratio=542.2±262.4 mm, R(2)=0.88±0.13, n=24), and right femurs (torsion/tension ratio=506.7±260.0 mm, R(2)=0.82±0.22, n=25). No statistically significant differences were found for external versus internal rotation groups or for left versus right femurs when comparing torsion/tension ratios (p=0.85) or R(2) values (p=0.54). A strongly coupled linear relationship between torsion and tension for human femurs was exhibited. This suggests an interplay between these two factors during activities of daily living and injury processes.

Effect of prior Salter or Chiari osteotomy on THA with developmental hip dysplasia.

BACKGROUND: Controversy exists regarding the outcome of THA after prior pelvic osteotomy. QUESTIONS/PURPOSES: We conducted a retrospective chart and radiographic review to obtain outcome measures for perioperative complications, acetabular and femoral component revisions, Harris hip score, and survivorship and compared these outcomes for patients presenting with developmental dysplasia of the hip treated surgically using THA with and without prior pelvic osteotomy. PATIENTS AND METHODS: We performed 103 primary THAs in 87 patients with osteoarthritis secondary to developmental dysplasia of the hip with a minimum 3-year followup. Previous pelvic osteotomy was performed in 52 hips (Salter, 40; Chiari, nine; Salter and Chiari, three), and 51 hips had no previous surgery (control group). RESULTS: The pelvic osteotomy group did not have higher rates of femoral or acetabular intraoperative fracture or dislocation compared with the control group. The overall revision rate was 28.8% in the pelvic osteotomy group compared with 19.6% in the control group. The revision rate for aseptic loosening was 23.1% in the pelvic osteotomy group compared with 17.6% in the control group. Harris hip scores (range, 20-87) were not compromised, and overall survivorship rates 8 years postoperatively were not different at any time between the pelvic osteotomy (83.3%) and control (88.4%) groups. CONCLUSIONS: Prior pelvic osteotomy did not lead to a higher perioperative complication rate, higher revision rate, compromised Harris hip score, or shortened survivorship in eventual THA in developmental dysplasia of the hip. LEVEL OF EVIDENCE: Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Function plateaus by one year in patients with surgically treated displaced midshaft clavicle fractures.

BACKGROUND: Based on short-term (1 year or less) followup, primary fixation of displaced midshaft clavicle fractures reportedly results in better function compared with that reported for nonoperative methods. Whether better function persists beyond 1 year is unclear. QUESTIONS/PURPOSES: For displaced midshaft clavicle fractures, do the better mean Disabilities of the Arm, Shoulder and Hand (DASH) and Constant-Murley Shoulder (CSS) scores for operative versus nonoperative treatment at 1 year change between 1- and 2-year followup? PATIENTS AND METHODS: We previously reported 132 patients in a randomized prospective trial at 1 year, and here we report a further followup of 95 of the 132 patients (72%) at 2 years after injury. We evaluated all patients with the DASH and CSS scores. RESULTS: The mean DASH and CSS scores were similar at 2 years compared with 1 year postinjury for both the nonoperated and operated patients. The mean scores for the operated patients remained higher than those in the nonoperative group (DASH operative 4.1 ± 7.0 versus DASH nonoperative 11.4 ± 19.7, CSS operative 97.1 ± 4.5 versus CSS nonoperative 91.6 ± 14.1) at 2 years postinjury. CONCLUSIONS: The improvement in DASH and CSS scores seen with primary fixation of displaced clavicle fractures persists at 2 years but does not differ from values seen after 1 year of followup, suggesting a clinical steady state has been reached whereby outcome is unlikely to change with time. LEVEL OF EVIDENCE: Level I, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

A biomechanical evaluation of press-fit stem constructs for tumor endoprosthetic reconstruction of the distal femur.

This study was designed to assess the biomechanical parameters of the older Kotz Modular Femur Tibia Reconstruction (Stryker Inc, Mahwah, NJ) stem and the newer Restoration and the unfluted Global Modular Replacement System (Stryker Inc, Mahwah, NJ) uncemented stems for use with tumor endoprostheses as well as to assess the optimal reaming technique for insertion of these stems. Fresh-frozen adult femora or composite distal femora were implanted with the uncemented stems. Separate experiments were performed to compare reaming technique and bone resection level. All constructs were mechanically tested for axial compression, lateral bending, and torsional stiffness and torque to failure. Results showed that the biomechanical performance of all the stems were similar with respect to each parameter. Cylindrical reaming was associated with a significantly higher torque to failure than flexible reaming in the diaphysis (P = .006). Newer uncemented stems provide adequate initial biomechanical stability for implantation in the distal femur.

A biomechanical comparison of two triple-screw methods for femoral neck fracture fixation in a synthetic bone model.

BACKGROUND: Fixation of femoral neck fractures is often accomplished by several screws inserted along the neck axis. Alignment and number of screws remain at the discretion of the surgeon. Two fracture repair methods were compared. METHODS: Sixteen large, left, adult, synthetic femurs, known as Third Generation Composite Femurs (Pacific Research Laboratories, Vashon, WA), were osteotomized with a transverse cut perpendicular to the neck axis. Fractures were reduced and repaired using method 1 (n = 8) or method 2 (n = 8) cannulated cancellous screw methods. Method 1 screws were inserted parallel in an upside-down triangle configuration and abutted against the cortical walls inferiorly, anteriorly, and posteriorly. Method 2 screws were placed adjacent to one another as often done clinically. Femoral shafts were positioned in a nonclinical vertical orientation to obtain conservative "lower bound" measurements. Specimens were tested for torsional and axial stiffness using subclinical loads followed by axial failure tests. RESULTS: Method 1 showed statistically higher values compared with method 2 for torsional stiffness (9.9 vs. 7.9 Nm/deg, method 1/method 2 ratio = 1.25, p = 0.018), axial stiffness (1469.0 vs. 1278.1 N/mm, method 1/method 2 ratio = 1.15, p = 0.023), and axial failure load (3493.5 vs. 2863.5 N, method 1/method 2 ratio = 1.22, p = 0.000). However, there were no statistical differences in axial failure displacement (10.9 vs. 16.9 mm, method 1/method 2 ratio = 0.64, p = 0.101) or axial failure energy (29.9 vs. 35.9 J, method 1/method 2 ratio = 0.83, p = 0.453). For both methods, femoral heads move distally while screw shafts cut through the spongy cancellous matrix of the femoral neck in knife-like fashion. CONCLUSIONS: Method 1 was more mechanically stable than method 2 in femoral neck fracture fixation as detected by three of five biomechanical measurements and equivalent to method 2 for two of five biomechanical measurements.

The biomechanics of locked plating for repairing proximal humerus fractures with or without medial cortical support.

BACKGROUND: Comminuted proximal humerus fracture fixation is controversial. Locked plate complications have been addressed by anatomic reduction or medial cortical support. The relative mechanical contributions of varus malalignment and lack of medial cortical support are presently assessed. METHODS: Forty synthetic humeri divided into three subgroups were osteotomized and fixed at 0 degrees, 10 degrees, and 20 degrees of varus malreduction with a locking proximal humerus plate (AxSOS, Global model; Stryker, Mahwah, NJ) to simulate mechanical medial support with cortical contact retained. Axial, torsional, and shear stiffness were measured. Half of the specimens in each of the three subgroups underwent a second osteotomy to create a segmental defect simulating loss of medial support with cortex removed. Axial, torsional, and shear stiffness tests were repeated, followed by shear load to failure in 20 degrees of abduction. RESULTS: For isolated malreduction with cortical contact, the construct at 0 degrees showed statistically equivalent or higher axial, torsional, and shear stiffness than other subgroups examined. Subsequent removal of cortical support in half the specimens showed a drastic effect on axial, torsional, and shear stiffness at all varus angulations. Constructs with cortical contact at 0 degrees and 10 degrees yielded mean shear failure forces of 12965.4 N and 9341.1 N, respectively, being statistically higher (p < 0.05) compared with most other subgroups tested. Specimens failed primarily by plate bending as the humeral head was pushed down medially and distally. CONCLUSIONS: Anatomic reduction with the medial cortical contact was the stiffest construct after a simulated two-part fracture. This study affirms the concept of medial cortical support by fixing proximal humeral fractures in varus, if absolutely necessary. This may be preferable to fixing the fracture in anatomic alignment when there is a medial fracture gap.

Immediate plate osteosynthesis of open fractures of the humeral shaft.

BACKGROUND: The aim of this study was to assess the outcome of immediate plate osteosynthesis in the surgical treatment of open humeral shaft fractures. METHODS: In a Level I trauma center and teaching hospital, we reviewed 53 patients with open humeral diaphyseal fractures who were treated with immediate open reduction and plate fixation from April 1988 to August 1998. Forty-six patients were available for adequate follow-up and assessment. All fractures were treated with a standard protocol that included irrigation and debridement, intravenous antibiotics, and immediate open reduction and plate fixation. Patients were assessed to determine whether bone grafts were needed during surgery, whether bone union was achieved, the state of the radial nerve, and postoperative complications. No quantitative functional outcomes were assessed, because this was beyond the scope of the study. It was hypothesized that immediate plate osteosynthesis would achieve safe and acceptable clinical results. RESULTS: The 46 patients with adequate final follow-up were assessed at a mean time of 37.5 weeks (13-156 weeks). All fractures united primarily in satisfactory angulation <5 degrees in coronal and sagittal planes. There were 6 delayed unions with a mean time to union of 42.5 weeks (30-72 weeks). The remaining 40 patients united at a mean of 18.4 weeks (12-26 weeks). No patient required subsequent surgery to obtain union of the fracture. Complications were rare, with no deep infections, nonunions, or iatrogenic nerve injuries. Complications included amputation in three patients and dysesthesia in one patient. Two patients had implants removed because of discomfort. CONCLUSIONS: Immediate plate osteosynthesis for open humeral shaft fractures has been shown to produce excellent results regarding bone union and absence of deep infections and is a safe technique in the management of these injuries.

Posttraumatic lung injury after pulmonary contusion and fat embolism: factors determining abnormal gas exchange.

BACKGROUND: The objective was to investigate changes in pulmonary blood flow after lung contusion and fat embolism. METHODS: Eighteen mongrel dogs were randomly assigned to three groups: fat embolism alone (n = 7); moderate unilateral pulmonary contusion followed by fat embolism (n = 6); and severe unilateral pulmonary contusion followed by fat embolism (n = 5). Fat embolism was produced by intramedullary reaming of left femur and tibia followed by canal pressurization using bone cement. Outcome measures were systemic blood pressure, pulmonary artery pressure, pulmonary artery occluded pressure, cardiac output (CO), and partial pressures of arterial and mixed venous oxygen (Pao2, PvO2). Samples were taken from contused and noncontused contralateral lung to calculate regional pulmonary blood flow. RESULTS: After the fat embolism, pulmonary artery pressure and pulmonary vascular resistance increased significantly (p < 0.05) in all groups, whereas Pao2 decreased in groups 2 and 3 and at 30 minutes in group 1. CO decreased significantly in group 3. Group 3 also demonstrated a greater initial decrease in Pao2 and PvO2 from baseline and a larger increase in pulmonary vascular resistance. In those animals that underwent contusion, regional pulmonary blood flow was not found to be different between contused and noncontused lung segments. After contusion, flow decreased significantly in contused and noncontused segments in group 3 only. CONCLUSIONS: Gas exchange deteriorates because of decreased CO. For any preexisting intrapulmonary shunt, the decrease of PvO2 will cause worsening of Pao2.

Development of the radiographic union score for tibial fractures for the assessment of tibial fracture healing after intramedullary fixation.

BACKGROUND: : The objective was to evaluate the newly developed Radiographic Union Score for Tibial fractures (RUST). Because there is no "gold standard," it was hypothesized that the RUST score would provide substantial improvements compared with previous scores presented in the literature. METHODS: : Forty-five sets of X-rays of tibial shaft fractures treated with intramedullary fixation were selected. Seven orthopedic reviewers independently scored bony union using RUST. Radiographs were reassessed at 9 weeks. Intraclass correlation coefficients (ICC) with 95% confidence intervals (CI) measured agreement. RESULTS: : Overall agreement was substantial (ICC, 0.86; 95% CI, 0.79-0.91). There was improved reliability among traumatologists compared with others (ICC = 0.86, 0.81, and 0.83, respectively). Overall intraobserver reliability was also substantial (ICC, 0.88; 95% CI, 0.80-0.96). CONCLUSIONS: : The RUST score exhibits substantial improvements in reliability from previously published scores and produces equally reproducible results among a variety of orthopedic specialties and experience levels. Because no "gold standards" currently exist against which RUST can be compared, this study provides only the initial step in the score's full validation for use in a clinical context.

The biomechanical consequence of insufficient femoral component lateralization and exposed cancellous bone in hip resurfacing arthroplasty.

Insufficient lateralization of the femoral component coupled with exposed reamed cancellous bone has been speculated to predispose to femoral neck fracture. The current study examined the effect of mediolateral implant position and exposed cancellous bone on the strength of the resurfaced proximal femur. Composite femurs were prepared in three configurations: (1) partial, with the implant placed at the native femoral head offset of the femur, partially exposing reamed cancellous bone; (2) proud, with a medialized implant exposing a circumferential ring of cancellous bone; and (3) complete, with a lateralized implant covering all reamed cancellous bone. Specimens were loaded to failure in axial compression. A finite element model was used to further explore the effect of exposed cancellous bone, cement mantle thickness, and relative valgus orientation on the strain distributions in the resurfaced femur. The proud group (2063 N) was significantly weaker than both the partial (2974 N, p=0.004) and complete groups (5899 N, p=0.001) when tested to failure. The partial group was also significantly weaker than the complete group when tested to failure (p=0.001). The finite element model demonstrated increasing levels of strain in the superior reamed cortical-cancellous bone interface with increasing degree of exposed cancellous bone. The condition of the femoral component medialized as the result of a thick cement mantle had the greatest detrimental impact on strain level in the superior reamed cancellous bone while a valgus oriented implant provided a protective effect. This study provides biomechanical evidence that exposed reamed cancellous bone significantly reduces the load-to-failure and increases maximum strains in the resurfaced proximal femur. The perceived benefit of reconstructing the femur to its native geometry may inherently weaken the proximal femur and increase femoral neck fracture risk if the femoral component is not sufficiently lateralized to cover all unsupported reamed cancellous bone. Relative valgus orientation of the implant may help to minimize the risk of neck fracture if reamed cancellous bone remains exposed following implant impaction.