Dynamic locking screw improves fixation strength in osteoporotic bone: an in vitro study on an artificial bone model.

PURPOSE: The novel dynamic locking screw (DLS) was developed to improve bone healing with locked-plate osteosynthesis by equalising construct stiffness at both cortices. Due to a theoretical damping effect, this modulated stiffness could be beneficial for fracture fixation in osteoporotic bone. Therefore, the mechanical behaviour of the DLS at the screw-bone interface was investigated in an artificial osteoporotic bone model and compared with conventional locking screws (LHS). METHODS: Osteoporotic surrogate bones were plated with either a DLS or a LHS construct consisting of two screws and cyclically axially loaded (8,500 cycles, amplitude 420 N, increase 2 mN/cycle). Construct stiffness, relative movement, axial screw migration, proximal (P) and distal (D) screw pullout force and loosening at the bone interface were determined and statistically evaluated. RESULTS: DLS constructs exhibited a higher screw pullout force of P 85 N [standard deviation (SD) 21] and D 93 N (SD 12) compared with LHS (P 62 N, SD 28, p = 0.1; D 57 N, SD 25, p < 0.01) and a significantly lower axial migration over cycles compared with LHS (p = 0.01). DLS constructs showed significantly lower axial construct stiffness (403 N/mm, SD 21, p < 0.01) and a significantly higher relative movement (1.1 mm, SD 0.05, p < 0.01) compared with LHS (529 N/mm, SD 27; 0.8 mm, SD 0.04). CONCLUSION: Based on the model data, the DLS principle might also improve in vivo plate fixation in osteoporotic bone, providing enhanced residual holding strength and reducing screw cutout. The influence of pin-sleeve abutment still needs to be investigated.

The locking attachment plate for proximal fixation of periprosthetic femur fractures--a biomechanical comparison of two techniques.

PURPOSE: Mechanical properties of a locking attachment plate construct (LAP-LCP), allowing bicortical screw placement laterally to the prosthesis stem, are compared to a cerclage-LCP construct. METHODS: Eight right synthetic femora with implanted uncemented hip endoprosthesis were cut distally and fixed with LCP, monocortical locking screws and either LAP (n = 4) or cerclage (n = 4). Cyclic testing was performed with monotonically increasing sinusoidal load until failure. Relative movements at the plate-femur interface were registered by motion tracking. Statistical differences were detected by unpaired t-test and general linear model repeated measures. RESULTS: Stiffness of the LAP-LCP was significantly higher at the beginning (875.4 N/mm ± 29.8) and after 5000 cycles (1213.0 N/mm ± 101.1) compared to the cerclage-LCP (644.96 N/mm ± 50.1 and 851.9 N/mm ± 81.9), with p = 0.013. Relative movements for AP-bending (B) and axial translation (T) of the LAP-LCP at the beginning (0.07° ± 0.02, 0.20 mm ± 0.08), after 500 cycles (0.16° ± 0.10, 0.26 mm ± 0.07) and after 5000 cycles (0.26° ± 0.11, 0.31 mm ± 0.07) differed significantly from the cerclage-LCP (beg.: 0.26° ± 0.04, 0.28 mm ± 0.05; 500 cyc: 0.47° ± 0.03, 0.53 mm ± 0.07; 5000 cyc.: 0.63° ± 0.18, 0.79 mm ± 0.13), with B: p = 0.02, T: p = 0.04. Relative movements for medial bending were not significantly different between the two constructs. Cycles to failure (criterion 1 mm axial translation) differed significantly between LAP-LCP (19,519 ± 1,758) and cerclage-LCP (11,265 ± 2,472), with p = 0.035. CONCLUSIONS: Biomechanically, the LAP-LCP construct improves proximal fixation of periprosthetic fractures compared to the cerclage-LCP construct.

Angulated locking plate in periprosthetic proximal femur fractures: biomechanical testing of a new prototype plate.

INTRODUCTION: To improve proximal plate fixation of periprosthetic femur fractures, a prototype locking plate with proximal posterior angulated screw positioning was developed and biomechanically tested. METHODS: Twelve fresh frozen, bone mineral density matched human femora, instrumented with cemented hip endoprosthesis were osteotomized simulating a Vancouver B1 fracture. Specimens were fixed proximally with monocortical (LCP) or angulated bicortical (A-LCP) head-locking screws. Biomechanical testing comprised quasi-static axial bending and torsion and cyclic axial loading until catastrophic failure with motion tracking. RESULTS: Axial bending and torsional stiffness of the A-LCP construct were (1,633 N/mm ± 548 standard deviation (SD); 0.75 Nm/deg ± 0.23 SD) at the beginning and (1,368 N/mm ± 650 SD; 0.67 Nm/deg ± 0.25 SD) after 10,000 cycles compared to the LCP construct (1,402 N/mm ± 272 SD; 0.54 Nm/deg ± 0.19 SD) at the beginning and (1,029 N/mm ± 387 SD; 0.45 Nm/deg ± 0.15) after 10,000 cycles. Relative movements for medial bending and axial translation differed significantly between the constructs after 5,000 cycles (A-LCP 2.09° ± 0.57 SD; LCP 5.02° ± 4.04 SD; p = 0.02; A-LCP 1.25 mm ± 0.33 SD; LCP 2.81 mm ± 2.32 SD; p = 0.02) and after 15,000 cycles (A-LCP 2.96° ± 0.70; LCP 6.52° ± 2.31; p = 0.01; A-LCP 1.68 mm ± 0.32; LCP 3.14 mm ± 0.68; p = 0.01). Cycles to failure (criterion 2 mm axial translation) differed significantly between A-LCP (15,500 ± 2,828 SD) and LCP construct (5,417 ± 7,236 SD), p = 0.03. CONCLUSION: Bicortical angulated screw positioning showed less interfragmentary osteotomy movement and improves osteosynthesis in periprosthetic fractures.

Double fixation for complex distal femoral fractures.

For complex distal femoral fractures, a single lateral locking compression plate or retrograde intramedullary nail may not achieve a stable environment for fracture healing. Various types of double fixation constructs have been featured in the current literature. Double-plate construct and nail-and-plate construct are two common double fixation constructs for distal femoral fractures. Double fixation constructs have been featured in studies on comminuted distal femoral fractures, distal femoral fracture with medial bone defects, periprosthetic fractures, and distal femoral non-union. A number of case series reported a generally high union rate and satisfactory functional outcomes for double fixation of distal femoral fractures. In this review, we present the state of the art of double fixation constructs for distal femoral fractures with a focus on double-plate and plate-and-nail constructs.

Effect on dynamic mechanical stability and interfragmentary movement of angle-stable locking of intramedullary nails in unstable distal tibia fractures: a biomechanical study.

BACKGROUND: Unstable distal tibia fractures are challenging injuries that require surgery. Increasingly, intramedullary nails are being used. However, fracture site anatomy may cause distal-fragment stabilization and fixation problems and lead to malunion/nonunion. We studied the influence of angle-stable nail locking on fracture gap movement and other biomechanical parameters. METHODS: Eight pairs of fresh human cadaver tibiae were used. The bone mineral density (BMD) was determined. All tibiae were nailed with a Synthes Expert tibial nail. Within each pair, one tibia was randomized to receive conventional locking screws; the other, angle-stable screws with sleeves. A 7-mm osteotomy was created 10 mm above the upper distal locking screw, to simulate an AO 42-A3 fracture. Biomechanical testing involved nondestructive mediolateral and anteroposterior pure bending, followed by cyclic combined axial and torsional loading to catastrophic failure. The neutral zone was determined. Fracture gap movement was monitored with 3-D motion tracking. RESULTS: The angle-stable locked constructs had a significantly smaller mediolateral neutral zone (mean: 0.04 degree; p=0.039) and significantly smaller fracture gap angulation (p=0.043). The number of cycles to failure did not differ significantly between the locking configurations. BMD was a significant covariate affecting the number of cycles to failure (p=0.008). However, over the first 20,000 cycles, there was no significant correlation in the angle-stable construct. CONCLUSIONS: Angle-stable locking of the Expert tibial nail was associated with a significant reduction in the mediolateral neutral zone and in fracture gap movement. Angle-stable fixation also reduced the influence of BMD over the first 20,000 cycles.

Risk of graft fracture after dorso-ventral thoraco-lumbar spondylodesis: is there a correlation with graft size?

STUDY DESIGN: Retrospective clinical study in patients with dorso-ventral thoraco-lumbar spondylodesis. OBJECTIVE: To investigate whether the ratio between graft cross sectional area and the surface area of the adjacent endplates has any effect on the midterm stability of the spondylodesis. Dorso-ventral spondylodesis in the region of the thoraco-lumbar spine is one of the most frequent operations in orthopaedic surgery. Anterior stabilization with autologous iliac crest graft currently is a standard approach in many hospitals. Although numerous recommendations are given how to perform this technique, no clinical advice is available with regard to minimum graft size. METHODS: Sixty-four-slice CT-scans were obtained from 82 patients 4-12 months after posterior spondylodesis with anterior implantation of iliac crest graft and stabilization with an internal fixator. The scans were analyzed using image analysis software. First, the cross sectional area of the graft was calculated and then the surface area of the adjacent endplates. The ratio between graft cross sectional area and endplate surface area was then calculated from these two values. The grafts were then evaluated in sagittal reconstruction for signs of fracture. RESULTS: The probability for graft fracture in autologous tricortical grafts was >0.1% (p < 0.001) if the graft cross sectional area exceeded 23.9% of the surface area of the adjacent endplates. Patients with lower ratio values had a higher fracture risk and below a value of 10% all grafts fractured. CONCLUSION: The relationship between graft cross sectional area and adjacent endplate area has an important effect on graft midterm stability in ventral spondylodesis of the thoraco-lumbar spine. In our opinion, the risk of graft fractures in dorso-ventral spondylodesis can be reduced by implantation of an appropriately sized graft without any additional procedures or instrumentation.

Transverse fractures of the olecranon: a biomechanical comparison of three fixation techniques.

INTRODUCTION: The gold standard for treating transverse olecranon fractures is tension band fixation. A problem with this technique is migration of the K-wires leading to premature hardware removal. The aim of this study is to compare stability provided by two new techniques designed to eliminate the problem with backing out of K-wires, with that of the recommended tension band technique, performed with a biomechanical in vitro investigation. Our hypothesis was that the two new techniques would provide at least equal stability as the traditional tension band fixation. METHODS: Transverse olecranon osteotomies were created in human cadaveric elbows to simulate a type 21-B1.1 fracture. Three groups of 8 specimens were instrumented with: (1) recommended AO tension band technique; (2) modified K-wires with eyelets and tension band; (3) staples across the fracture with tension band. Each elbow was tested in a 90° flexed position. The triceps tendon was sinusoidally loaded by applying two load steps at 500 and 700 N for 4000 cycles each. Relative movements between the fragments were determined. RESULTS: At the end of the first and second load step the displacement of the osteotomy at the posterior ulnar side was significantly less for the staples across the fracture with tension band as compared to both other groups. There were no significant differences between groups 1 and 2. CONCLUSION: Since clinical results depend partly on stable fixation, it is concluded that using staples in the clinical situation might provide better results than the currently recommended tension band technique.

Treatment of distal humeral fractures using conventional implants. Biomechanical evaluation of a new implant configuration.

BACKGROUND: In the face of costly fixation hardware with varying performance for treatment of distal humeral fractures, a novel technique (U-Frame) is proposed using conventional implants in a 180 degrees plate arrangement. In this in-vitro study the biomechanical stability of this method was compared with the established technique which utilizes angular stable locking compression plates (LCP) in a 90 degrees configuration. METHODS: An unstable distal 3-part fracture (AO 13-C2.3) was created in eight pairs of human cadaveric humeri. All bone pairs were operated with either the "Frame" technique, where two parallel plates are distally interconnected, or with the LCP technique. The specimens were cyclically loaded in simulated flexion and extension of the arm until failure of the construct occurred. Motion of all fragments was tracked by means of optical motion capturing. Construct stiffness and cycles to failure were identified for all specimens. RESULTS: Compared to the LCP constructs, the "Frame" technique revealed significant higher construct stiffness in extension of the arm (P = 0.01). The stiffness in flexion was not significantly different (P = 0.16). Number of cycles to failure was found significantly larger for the "Frame" technique (P = 0.01). CONCLUSIONS: In an in-vitro context the proposed method offers enhanced biomechanical stability and at the same time significantly reduces implant costs.

Biomechanical investigation of an alternative concept to angular stable plating using conventional fixation hardware.

BACKGROUND: Angle-stable locking plates have improved the surgical management of fractures. However, locking implants are costly and removal can be difficult. The aim of this in vitro study was to evaluate the biomechanical performance of a newly proposed crossed-screw concept ("Fence") utilizing conventional (non-locked) implants in comparison to conventional LC-DCP (limited contact dynamic compression plate) and LCP (locking compression plate) stabilization, in a human cadaveric diaphyseal gap model. METHODS: In eight pairs of human cadaveric femora, one femur per pair was randomly assigned to receive a Fence construct with either elevated or non-elevated plate, while the contralateral femur received either an LCP or LC-DCP instrumentation. Fracture gap motion and fatigue performance under cyclic loading was evaluated successively in axial compression and in torsion. Results were statistically compared in a pairwise setting. RESULTS: The elevated Fence constructs allowed significantly higher gap motion compared to the LCP instrumentations (axial compression: p

Ankle joint pressure changes in a pes cavovarus model after lateralizing calcaneal osteotomies.

BACKGROUND: Tendon transfers and calcaneal osteotomies are commonly used to treat symptoms related to medial ankle arthrosis in fixed pes cavovarus. However, the relative effect of these osteotomies in terms of lateralizing the ground contact point of the hindfoot and redistributing ankle joint contact stresses are unknown. MATERIALS AND METHODS: Pes cavovarus with fixed hindfoot varus was simulated in eight cadaver specimens. The effect of three types of calcaneal osteotomies on the migration of the center of force and tibiotalar peak pressure at 300 N axial static load (half-body weight) were recorded using pressure sensors. RESULTS: A significant lateral shift of the center of force was observed: 4.9 mm for the laterally closing Z-shaped osteotomy with additional lateralization of the tuberosity, 3.4 mm for the lateral sliding osteotomy of the calcaneal tuberosity, and 2.7 mm for the laterally closing Z-shaped osteotomy (all p < 0.001). A significant peak pressure reduction was recorded: -0.53 MPa for the Z-shaped osteotomy with lateralization, -0.58 MPa for the lateral sliding osteotomy of the calcaneal tuberosity, and -0.41 MPa for the Z-shaped osteotomy (all p < 0.01). CONCLUSION: This cadaver study supports the hypothesis that lateralizing calcaneal osteotomies substantially help to normalize ankle contact stresses in pes cavovarus.

Stability of medial locking plate and compression screw versus two crossed screws for lapidus arthrodesis.

BACKGROUND: Lapidus (first metatarsocuneiform joint) arthrodesis is an established procedure for the management of hallux valgus. This study investigated the utility of fixation with a medial locking plate with adjunct compression screw versus fixation with two crossed screws. MATERIALS AND METHODS: Eight pairs of fresh-frozen human specimens were used in a matched pair test. Bone mineral density (BMD) was measured with peripheral quantitative computed tomography (pQCT). Fixation with two 4-mm-diameter crossed screws was compared versus a medial locking plate (X-Locking Plate 2.4/2.7; Synthes, Solothurn, Switzerland) with adjunct 4-mm-diameter compression screw. The specimens were tested in a four-point bending test. Parameters obtained were initial stiffness; plantar joint-line gapping after one cycle, 100 and 1000 cycles; and number of cycles to failure. Failure was defined as more than or equal to 3 mm plantar gapping. RESULTS: The groups did not differ significantly with regard to BMD (p = 0.866) and initial stiffness (p = 0.889). The plate-and-screw construct showed significantly less movement during testing, and significantly (p = 0.001) more cycles to failure than did the crossed-screw construct. There was a significant correlation (crossed-screw construct: p = 0.014; plate-and-screw construct: p = 0.010) between BMD and the number of cycles to failure. CONCLUSION: Under cyclic loading conditions, the construct using a medial locking plate with adjunct compression screw was superior to the construct using two crossed screws. CLINICAL RELEVANCE: The medial locking-plate technique described could help shorten the period of nonweightbearing and reduce the risk of non-union.

In vitro mechanical evaluation of a novel pin-sleeve system for external fixation of distal limb fractures in horses: a proof of concept study.

OBJECTIVE: To evaluate the efficacy of a novel pin-sleeve cast (PSC) system for external fixation of distal limb fractures in horses and to compare it with the transfixation pin cast (TPC) system. STUDY DESIGN: Experimental. SAMPLE POPULATION: One bone substitute each was used for the TPC and PSC systems. The PSC was tested in 4 configurations characterized by different pin preloads. METHODS: Specimens were loaded in axial compression in the elastic range. Variables compared statistically were: bone substitute axial displacement and axial strain measured above implants with strain gauges. Pin preload was correlated with the variables investigated. Load to failure and a fatigue tests supplemented the investigation. RESULTS: The PSC configuration with the highest pin preload showed a significantly lower axial displacement compared with the TPC. No significant differences were observed between all other PSC configurations and the TPC. All PSC systems had a significant decrease in recorded strain compared with the TPC system. Pin axial preload inversely correlated with axial displacement but had no effect on axial strain. In the failure test, the PSC encountered plastic deformation earlier than the TPC. In the fatigue test, the PSC ran >200,000 cycles. CONCLUSIONS: Preliminary in vitro tests showed that the PSC system significantly reduced peri-implant strain while concurrently having comparable axial displacement to the TPC system. CLINICAL RELEVANCE: The PSC system has the potential to reduce the risk of pin loosening in horses.

Intraoperative mechanical bone strength determination in tibiotalocalcaneal fusion: a biomechanical investigation.

BACKGROUND: Bone strength is currently measured with indirect techniques. We investigated the use of an intraoperative mechanical measurement for local bone strength determination and prediction of intramedullary-nail fusion failure. We investigated whether intraoperative local bone strength determination may be useful to the surgeon in predicting intramedullary nail hindfoot fusion performance. MATERIALS AND METHODS: In seven human specimens, bone mineral density (BMD) was determined with qCT. A device (DensiProbe) specially devised for nailed tibiotalocalcaneal arthrodesis (TTCA) was inserted at the intended calcaneal screw sites of an intramedullary nail, and the cancellous break-away torque was measured. The constructs were then cyclically loaded to failure in dorsiflexion-plantarfexion. RESULTS: The BMD range was wide (42.8 to 185.9 mg HA/cm(3)). The proximal-screw site peak torque was 0.47 to 1.61 Nm; distal-screw site peak torque was 0.24 to 1.06 Nm. The number of cycles to failure correlated with peak torque both proximally (p = 0.021; r(2) = 0.69) and distally (p = 0.001; r(2) = 0.92). Proximally, peak torque did not correlate with BMD (p = 0.060; r(2) = 0.54); distally, it correlated significantly (p = 0.003; r(2) = 0.86). CONCLUSION: DensiProbe measurements can be used in the hindfoot to assess bone strength. In this study, specimens that failed early could be identified. However, in clinical practice fusion failure is multifactorial in origin, and failure prediction cannot be based upon peak torque measurements alone. CLINICAL RELEVANCE: The technique described here may be of use to give an intraoperative decision aid to predict intramedullary nail hindfoot fusion performance.

Comparison of calcaneal fixation of a retrograde intramedullary nail with a fixed-angle spiral blade versus a fixed-angle screw.

BACKGROUND: Retrograde intramedullary nailing is an established technique for tibiotalocalcaneal arthrodesis (TTCA). In poor bone stock (osteoporosis, neuroarthropathy), device fixation in the hindfoot remains a problem. Fixed-angle spiral-blade fixation of the nail in the calcaneus could be useful. MATERIALS AND METHODS: In seven matched pairs of human below-knee specimens, bone mineral density (BMD) was determined, and TTCA was performed with an intramedullary nail (Synthes Hindfoot Arthrodesis Nail HAN Expert Nailing System), using a conventional screw plus a fixed-angle spiral blade versus a conventional screw plus a fixed-angle screw, in the calcaneus. The constructs were subjected to quasi-static loading (dorsiflexion/plantarflexion, varus/valgus, rotation) and to cyclic loading to failure. Parameters studied were construct neutral zone (NZ) and range of motion (ROM), and number of cycles to failure. RESULTS: With dorsiflexion/plantarflexion loading, the screw-plus-spiral-blade constructs had a significantly smaller ROM in the quasi-static test (p = 0.028) and early in the cyclic test (p = 0.02); differences in the other parameters were not significant. There was a significant correlation between BMD and cycles to failure for the two-screw constructs (r = 0.94; p = 0.002) and for the screw-plus-spiral-blade constructs (r = 0.86; p = 0.014). CONCLUSION: In TTCA with a HAN Expert Nailing System, the use of a calcaneal spiral blade can further reduce motion within the construct. This may be especially useful in poor bone stock. CLINICAL RELEVANCE: Results obtained in this study could be used to guide the operating surgeon's TTCA strategy.

Mechanical comparison in cadaver specimens of three different 90-degree double-plate osteosyntheses for simulated C2-type distal humerus fractures with varying bone densities.

OBJECTIVES: To investigate the bone-implant-anchorage of 90-degree double-plate osteosynthesis in simulated complete intra-articular distal humerus fractures using conventional reconstruction plates (CRP), locking compression plates (LCP), and distal humerus plates (DHP), depending on the bone mineral density (BMD) of the cadaver specimens. METHODS: Groups (CRP, LCP, DHP, n=8; LCP, DHP, n=13) in distal humerus cadaver bones were created based on BMD. The fracture model was an unstable intraarticular distal humerus fracture with a transverse osteotomy gap representing metaphyseal comminution (AO type 13-C2.3). Flexion and extension stiffness as well as cycles until failure due to screw pullout under cyclic loading were evaluated. Estimates of BMD values, below which failure was likely to occur, were determined. RESULTS: Stiffness values were not significantly different between groups (extension: P=0.881, flexion: P=0.547). Under cyclic loading, consistent screw pullout failure occurred at BMD values below about 400 mg/cm for CRP and below about 300 mg/cm for LCP constructs. Comparing BMD-matched groups of 8 and 13 specimens respectively, the failure rate was significantly lower for the DHP (0/8) than for the CRP (5/8; P=0.026) and tended to be lower for the DHP (0/13) as compared to the LCP (4/13; P=0.096). CONCLUSION: Bone-implant anchorage was different between locking and nonlocking plate constructs and depended on BMD. While in good bone quality implant choice was not critical, both locking plates provided superior resistance against screw loosening as compared to the CRP at low BMD values (<420 mg/cm). Based on our laboratory results, we conclude that locking plates such as the LCP and DHP are constructs designed to keep anatomical reduction in the presence of comminution and poor bone quality in a low intra-articular fracture of the distal humerus.

Comparison of locking and conventional screws for maintenance of tibial plateau positioning and biomechanical stability after locking tibial plateau leveling osteotomy plate fixation.

OBJECTIVE: To compare locking screws with conventional screws inserted in the tibial plateau fragment for reduction and stability of the construct after tibial plateau leveling osteotomy (TPLO), using a locking TPLO plate. STUDY DESIGN: Experimental biomechanical study. ANIMALS: Cadaveric canine pelvic limbs (n=8 pairs). METHODS: TPLO was stabilized with either conventional cortical screws or locking screws in a compressed osteotomy model. Titanium pins inserted into the tibial plateau and proximal metaphysis were used to track bone fragment location by computed tomography (CT) imaging. CT imaging was performed after osteotomy reduction, after plate stabilization, and after 30,000 cycles of axial compression testing. After 30,000 cycles, cyclic loading was continued with monotonically increasing peak-load until failure. RESULTS: The magnitude of rotation about the sawing axis was significantly greater for the conventional screw group because of plate application (P=.009). Translational movement of the tibial plateau fragment toward the plate was significantly greater for the conventional screw group (P=.006). There were no significant differences between groups in stiffness or number of cycles to failure. CONCLUSION: Maintenance of tibial plateau position was significantly superior for the locking screw group during plate application; however, screw type had no effect on fixation stability under cyclic loading. CLINICAL RELEVANCE: These results suggest that conventional screws and careful contouring of the TPLO plate can provide comparable mechanical stability to fixation with locking screws in the tibial plateau under load-sharing conditions, but potentially at the expense of osteotomy reduction.

Accuracy of fragment positioning after TPLO and effect on biomechanical stability.

OBJECTIVE: To compare tibial plateau rotation after tibial plateau leveling osteotomy with the radiographically planned rotation and to determine the effect of translations and rotations of the tibial plateau fragment on the biomechanical stability of the construct under cyclic loading. STUDY DESIGN: Experimental biomechanical study. ANIMALS: Cadaveric canine pelvic limbs (n=10). METHODS: Titanium pins were inserted into the tibial plateau and the proximal metaphysis to track the fragment movements by means of computed tomography (CT) imaging. CT scans were performed (1) before osteotomy, (2) after osteotomy and tibial plateau rotation, and (3) after stabilization with plate and screws. The bones were then cyclically loaded in axial compression. RESULTS: The radiographically planned tibial plateau rotation correlated significantly with the achieved rotation (r=0.73, P=.016), although deviations of up to 4.7 degrees were observed. A significant positive correlation between the amount of rotation about the sawing axis and the plastic deformation of the construct after 30,000 test cycles could be found (r=0.81, P=.005). CONCLUSION: Considerable deviation occurred between planned and achieved rotation of the tibial plateau fragment. Lower degrees of rotation were beneficial for biomechanical stability. CLINICAL RELEVANCE: Dogs with larger tibial plateau angles may be at a relatively higher risk for fixation failure, but further studies are needed to establish a safe margin of tibial plateau rotation.

Comparison of robotic-assisted and manual implantation of a primary total hip replacement. A prospective study.

BACKGROUND: Robotic-assisted total hip replacement has become a common method of implantation, especially in Europe. It frequently has been postulated that robotic reaming would result in an improved clinical outcome due to the better fit of the prosthesis, but that has never been demonstrated in a prospective study, to our knowledge. The purpose of this study was to compare robotic-assisted implantation of a total hip replacement with conventional manual implantation. METHODS: One hundred and fifty-four patients scheduled for total hip replacement were randomly assigned to undergo either conventional manual implantation of an S-ROM prosthesis (eighty patients) or robotic-assisted implantation of such a prosthesis (seventy-four patients). The five-axis ROBODOC was used for the robotic-assisted procedures. Preoperatively as well as at three, six, twelve, and twenty-four months after surgery, the scores according to the Harris and Merle d'Aubigné systems and the Mayo clinical score were determined. Radiographs made at these intervals were analyzed for evidence of loosening, prosthetic alignment, and heterotopic ossification. RESULTS: Thirteen (18%) of the seventy-four attempted robotic implantations had to be converted to manual implantations as a result of failure of the system. The duration of the robotic procedures was longer than that of the manual procedures (mean and standard deviation,107.1 +/- 29.1 compared with 82.4 +/- 23.4 minutes, p < 0.001). Limb-length equality (mean discrepancy, 0.18 +/- 0.30 compared with 0.96 +/- 0.93 cm, p < 0.001) and varus-valgus orientation of the stem (mean angle between the femur and the shaft of the prosthesis, 0.34 degrees +/- 0.67 degrees compared with 0.84 degrees +/- 1.23 degrees, p < 0.001) were better after the robotic procedures. At six months, slightly more heterotopic ossification was seen in the group treated with robotic implantation. The group treated with robotic implantation had a better Mayo clinical score at six and twelve months and a better Harris score at twelve months; however, by twenty-four months, no difference was found between the groups with regard to any of the three scores. Dislocation was more frequent in the group treated with robotic implantation: it occurred in eleven of the sixty-one patients in that group compared with three of eighty in the other group (p < 0.001). Recurrent dislocation and pronounced limping were indications for revision surgery in eight of the sixty-one patients treated with robotic implantation compared with none of the seventy-eight (excluding two with revision for infection) treated with manual insertion (p < 0.001). Rupture of the gluteus medius tendon was observed during all of the revision operations. CONCLUSIONS: The robotic-assisted technology had advantages in terms of preoperative planning and the accuracy of the intraoperative procedure. Disadvantages were the high revision rate; the amount of muscle damage, which we believe was responsible for the higher dislocation rate; and the longer duration of surgery. This technology must be further developed before its widespread usage can be justified.

Abrasive water jet cutting as a new procedure for cutting cancellous bone--in vitro testing in comparison with the oscillating saw.

The quality of bone cuts is assessed by the accuracy and biological potency of the cut surfaces. Conventional tools (such as saws and milling machines) can cause thermal damage to bone tissue. Water jet cutting is nonthermal; that is, it does not generate heat. This study investigates whether the abrasive jet cutting quality in cancellous bone with a biocompatible abrasive is sufficient for the implantation of endoprostheses or for osteotomies. Sixty porcine femoral condyles were cut with an abrasive water jet and with an oscillating saw. alpha-lactose-monohydrate was used as a biocompatible abrasive. Water pressure (pW = 35 and 70 MPa) and abrasive feed rate (m = 0.5, 1, and 2 g/s) were varied. As a measure of the quality of the cut surface the cutting gap angle (delta) and the surface roughness (Ra) were determined. The surface roughness was lowest for an abrasive feed rate of m = 2 g/s (jet direction: 39 +/- 16 microm, advance direction: 54 +/- 22 microm). However, this was still significantly higher than the surface roughness for the saw group (jet direction: 28 +/- 12 microm, advance direction: 36 +/- 19 microm) (p < 0.001 for both directions). At both pressure levels the greatest cutting gap angle was observed for a mass flow rate of m = 1 g/s (pW = 35 MPa: delta = 2.40 +/- 4.67 degrees ; pW = 70 MPa: delta = 4.13 +/- 4.65 degrees), which was greater than for m = 0.5 g/s (pW = 35 MPa: delta = 1.63 +/- 3.89 degrees ; pW = 70 MPa: delta = 0.36 +/- 1.70 degrees) and m = 2 g/s (pW =70 MPa: delta = 0.06 +/- 2.40 degrees). Abrasive water jets are suitable for cutting cancellous bone. The large variation of the cutting gap angle is, however, unfavorable, as the jet direction cannot be adjusted by a predefined value. If it is possible to improve the cutting quality by a further parameter optimization, the abrasive water jet may be the cutting technique of the future for robotic usage.

The water jet as a new tool for endoprosthesis revision surgery--an in vitro study on human bone and bone cement.

In revision surgeries of endoprostheses, the interface between implant and bone cement or bone must be loosened. Conventional tools have many disadvantages because of their size and limited range. Taking advantage of the selective and athermic cutting process, a plain water jet is already used in order to cut soft tissues. This study investigates the possibilities of both a plain and an abrasive water jet as cutting tools for revision surgery. Samples of the mid-diaphysis of human femora and bone cement (CMW3) were cut with a plain water jet (PWJ) and an abrasive water jet (AWJ) at two different jet-to-surface angles (30 degrees,90 degrees ) and at five different pressure levels (30, 40, 50, 60, 70 MPa). For a PWJ a selective pressure range was identified, where only bone cement was cut. Injecting a bio-compatible abrasive (lactose) to the jet stream resulted in significantly higher cut depths in both materials. Material removal in bone was significantly less at the smaller jet-to-surface angle for both techniques. No clear selectivity between bone and bone cement was observed for application of the AWJ. However, the material removal rate was significantly higher for bone cement than for bone at all pressure levels. The results indicate that an AWJ might be an alternative tool for cement removal. The possibility for localised cutting at interfaces could be an advantage for revision of a non-cemented prosthesis.