Implant Material, Type of Fixation at the Shaft, and Position of Plate Modify Biomechanics of Distal Femur Plate Osteosynthesis.

OBJECTIVES: To investigate whether (1) the type of fixation at the shaft (hybrid vs. locking), (2) the position of the plate (offset vs. contact) and (3) the implant material has a significant effect on (a) construct stiffness and (b) fatigue life in a distal femur extraarticular comminuted fracture model using the same design of distal femur periarticular locking plate. METHODS: An extraarticular severely comminuted distal femoral fracture pattern (OTA/AO 33-A3) was simulated using artificial bone substitutes. Ten-hole distal lateral femur locking plates were used for fixation per the recommended surgical technique. At the distal metaphyseal fragment, all possible locking screws were placed. For the proximal diaphyseal fragment, different types of screws were used to create 4 different fixation constructs: (1) stainless steel hybrid (SSH), (2) stainless steel locked (SSL), (3) titanium locked (TiL), and (4) stainless steel locked with 5-mm offset at the diaphysis (SSLO). Six specimens of each construct configuration were tested. First, each specimen was nondestructively loaded axially to determine the stiffness. Then, each specimen was cyclically loaded with increasing load levels until failure. RESULTS: Construct Stiffness: The fixation construct with a stainless steel plate and hybrid fixation (SSH) had the highest stiffness followed by the construct with a stainless steel plate and locking screws (SSL) and were not statistically different from each other. Offset placement (SSLO) and using a titanium implant (TiL) significantly reduced construct stiffness. Fatigue Failure: The stainless steel with hybrid fixation group (SSH) withstood the most number of cycles to failure and higher loads, followed by the stainless steel plate and locking screw group (SSL), stainless steel plate with locking screws and offset group (SSLO), and the titanium plate and locking screws group (TiL) consecutively. Offset placement (SSLO) as well as using a titanium implant (TiL) reduced cycles to failure. CONCLUSIONS: Using the same plate design, the study showed that implant material, screw type, and position of the plate affect the construct stiffness and fatigue life of the fixation construct. With this knowledge, the surgeon can decide the optimal construct based on a given fracture pattern, bone strength, and reduction quality.

External fixation design evolution enhances biomechanical frame performance.

BACKGROUND: External fixation has become a quick and easy application for fracture stabilisation of the extremities and/or pelvis to maintain the reduction and provide stability while sparing the soft tissues. Over the last years, enhanced construct stiffness has become an essential requirement to preserve fracture reduction, particularly in active and overweight patients. This study was performed to determine whether the advancement of design features enhances the external fixation construct stiffness. The stiffness of the recently developed Hoffmann 3 external fixation system was determined and its characteristics compared with the widely clinically accepted Hoffmann II MRI fixation system. METHODS: A synthetic fracture model was used. Two carbon tubes with a fracture gap of 20 mm were appropriate to determine the stiffness of three different configurations: the basic frame configuration (group H 3, representing Hoffmann 3 with a rod diameter of 11 mm) using a double rod construction with 6 mm Apex pins, was compared with the Hoffmann II MRI fixation system using two 8.0 mm diameter rods with 6 mm (group H II-6 mm) and 5 mm (group H II-5 mm) Apex pins. Each group was tested five times under anterior-posterior bending (N/mm), medio-lateral bending (N/mm) and axial torsion loading directions (Nm/deg). The stiffness results of each construct were compared statistically. RESULTS: The basic frame construct (group H 3) showed consistently higher stiffness properties compared with the other configurations. The anterior-posterior-bending loads resulted in a mean value of 31 N/mm, which was significantly higher compared with the other groups (p=0.008) at 16 N/mm. The medio-lateral-bending test revealed a mean stiffness of 59 N/mm in the H3 group, compared with 43 N/mm in the H II-6 group and 31 N/mm in the H II-5 group. The axial torsion measurements of the Hoffmann 3 group yielded significantly higher results (1.03 Nm/°) compared with group H II-6 (0.61 Nm/°) and group H II-5 (0.56 Nm/°). CONCLUSIONS: The Hoffmann 3 construct showed the highest stiffness properties under bending and torsion loads. The enhanced stiffness of the Hoffmann 3 device may be helpful in maintaining fracture reduction and soft tissue compromise. This investigation showed the advancement of Hoffmann design features may be effective in enhancing frame stiffness.

Effect of angular stability and other locking parameters on the mechanical performance of intramedullary nails.

To extend the indications of intramedullary nails for distal or proximal fractures, nails with angle stable locking options have been developed. Studies on the mechanical efficacy of these systems have been inconsistent likely due to confounding variables such as number, geometry, or orientation of the screws, as well as differences in the loading mode. Therefore, the aim of this study was to quantify the effect of angular stability on the mechanical performance of intramedullary nails. The results could then be compared with the effects of various locking screw parameters and loading modes. A generic model was developed consisting of artificial bone material and titanium intramedullary nail that provided the option to systematically modify the locking screw configuration. Using a base configuration, the following parameters were varied: number of screws, distance and orientation between screws, blocking of screws, and simulation of freehand locking. Tension/compression, torsional, and bending loads were applied. Stiffness and clearance around the zero loading point were determined. Angular stability had no effect on stiffness but completely blocked axial clearance (p=0.003). Simulation of freehand locking reduced clearance for all loading modes by at least 70% (p<0.003). The greatest increases in torsional and bending stiffness were obtained by increasing the number of locking screws (up to 80%, p<0.001) and by increasing the distance between them (up to 70%, p<0.001). In conclusion, our results demonstrate that the mechanical performance of IM nailing can be affected by various locking parameters of which angular stability is only one. While angular stability clearly reduces clearance of the screw within the nail, mechanical stiffness depends more on the number of screws and their relative distance. Thus, optimal mechanical performance in IM nailing could potentially be obtained by combining angular stability with optimal arrangement of locking screws.

Complex distal humerus fractures-comparison of polyaxial locking and nonlocking screw configurations-a preliminary biomechanical study.

OBJECTIVE: The investigation hypothesized that in current anatomical precontoured plates, angular stability plays only a minor role for the efficacy of the osteosynthesis at the distal humerus. METHODS: An AO C2.3 fracture model was simulated and osteosynthesis performed with plates positioned in parallel. System rigidity and median fatigue limit were analyzed in artificial bones and the cycles to failure in cadaver specimens. Loads were applied in anterior-posterior direction (75° flexion) and axial direction (5° flexion). Four composite bone groups were investigated as follows: (1) 2.7 mm polyaxial locking screws, (2) 3.5 mm polyaxial locking screws, (3) 3.5 mm polyaxial locking screws and a gap bridging screw, and (4) 2.7 mm nonlocking screws. Two cadaver groups were investigated with 3.5 mm diameter polyaxial locking (5) versus nonlocking screws (6). RESULTS: There were no differences in stiffness found between the locking versus nonlocking constructs in artificial (1) versus (4) and in cadaver bones (5) versus (6). The larger screw diameter of 3.5 mm in combination with a gap bridging screw significantly increased construct stiffness by 25% (3). The median fatigue limit was significantly increased using larger screw diameters (2) and a gap bridging screw (3). In cadaver bones, the polyaxial locking screws constructs (5) resisted higher peak loads and more cycles until failure compared with nonlocking constructs (6). CONCLUSIONS: System stiffness increases with larger screw diameters and becomes significant with additional gap bridging screws in artificial bones. The use of polyaxial locking screws in anatomical adapted plates becomes more important in poor bone quality.

Evaluation of risk for secondary fracture after removal of a new femoral neck plate for intracapsular hip fractures.

OBJECTIVE: To determine whether a new femoral neck plate has a higher risk for secondary fracture after implant removal than the current standard treatment for intracapsular hip fractures. METHODS: Six pairs of human cadaver femora (age, 56 ± 5.6 years; range, 48-64 years; two female and four male donors) were instrumented with the femoral neck plate (FNP) or the compression hip screw combined with an antirotation screw (CHS) in a paired study design. After removal of the implants, axial compression tests to failure of the bones were conducted. Maximum force to failure of the bones after implant removal was determined. Axial stiffness of the bones before surgery and after implant removal was determined. RESULTS: The FNP resulted in a mean failure load of 4687 ± 1743 N (mean ± standard deviation) and the CHS resulted in a mean failure load of 4892 ± 1608 N with no significant difference between the two implant groups (P = 0.405). There was no significant difference in stiffness (P = 0.214) between the FNP (1240 ± 362 N/mm) and the CHS (1293 ± 304 N/mm). The cavities left by the surgery had no effect on the bone stiffness (P > 0.05). The mean failure load of all specimens correlated with the bone mineral density in the proximal part of the femurs by R² = 0.715 (P = 0.001). CONCLUSION: The FNP demonstrated a similar failure load after implant removal compared with the CHS, although the FNP left a 39% larger cavity in the bone.

Internal fixation of type-C distal femoral fractures in osteoporotic bone: surgical technique.

BACKGROUND: Fixation of distal femoral fractures remains a challenge, especially in osteoporotic bone. This study was performed to investigate the biomechanical stability of four different fixation devices for the treatment of comminuted distal femoral fractures in osteoporotic bone. METHODS: Four fixation devices were investigated biomechanically under torsional and axial loading. Three intramedullary nails, differing in the mechanism of distal locking (with two lateral-to-medial screws in one construct, one screw and one spiral blade in another construct, and four screws [two oblique and two lateral-to-medial with medial nuts] in the third), and one angular stable plate were used. All constructs were tested in an osteoporotic synthetic bone model of an AO/ASIF type 33-C2 fracture. Two nail constructs (the one-screw and spiral blade construct and the four-screw construct) were also compared under axial loading in eight pairs of fresh-frozen human cadaveric femora. RESULTS: The angular stable plate constructs had significantly higher torsional stiffness than the other constructs; the intramedullary nail with four-screw distal locking achieved nearly comparable results. Furthermore, the four-screw distal locking construct had the greatest torsional strength. Axial stiffness was also the highest for the four-screw distal locking device; the lowest values were achieved with the angular stable plate. The ranking of the constructs for axial cycles to failure was the four-screw locking construct, with the highest number of cycles, followed by the angular stable plate, the spiral blade construct, and two-screw fixation. The findings in the human cadaveric bone were comparable with those in the synthetic bone model. Failure modes under cyclic axial load were comparable for the synthetic and human bone models. CONCLUSIONS: The findings of this study support the concept that, for intramedullary nails, the kind of distal interlocking pattern affects the stabilization of distal femoral fractures. Four-screw distal locking provides the highest axial stability and nearly comparable torsional stability to that of the angular stable plate; the four-screw distal interlocking construct was found to have the best combined (torsional and axial) biomechanical stability.

Should extramedullary fixations for hip fractures be removed after bone union?

BACKGROUND: Osteosynthesis implants, which remain in the patient after fracture union to save additional surgery, may affect the strain distribution within the bone. A reduction of strain within the bone is known to result in localized bone loss ("stress shielding") and increased fracture risk. The purpose of this study was to examine whether extramedullary fixations for femoral neck fractures have to be removed after fracture union to prevent reductions in cortex strains. METHODS: In a biomechanical experiment, six pairs of human cadaver femora (mean age 56 years, range 48 to 64) were supplied with five strain gauges per bone. The bones were equally supplied with a compression hip screw or a femoral neck plate. Before surgery, after surgery and after removal of the implants, axial compression tests were conducted to measure surface strains during loading. FINDINGS: The compression hip screw reduced the amount of strain at the superior neck by 88% (P=0.015) and at the lesser trochanter by 51% (P=0.038). The femoral neck plate reduced the amount of strain at the superior neck by 89% (P=0.001), and increased the amount of strain at the inferior neck by 58% (P=0.02) and at the lesser trochanter by 63% (P=0.005). After implant removal, there was no significant difference in strain compared to pre-fracture levels, except for the compression hip screw with 21% less strain (P=0.047) at the superior neck. INTERPRETATION: Removal of osteosynthesis implants after bone union reverts bone strains to pre-fracture levels, and might prevent further bone loss induced by stress shielding.

Hip screw migration testing: first results for hip screws and helical blades utilizing a new oscillating test method.

Despite continued improvement in the methods and devices used to treat intertrochanteric fractures, there remains an unacceptable amount of failures. The cut-out rate for hip screws has been recorded up to 8.3%. To evaluate the migration of different implants under physiological loads, a multiplanar biomechanical test method for hip screws was developed, the first to incorporate a simulation of the human gait cycle by an oscillating flexion/extension movement of the test device. The new method was used to compare different hip screw and blade designs with respect to their directional migration resistance. The test method generated failure modes that were consistent with those observed clinically. Under cyclic loading, the hip screws migrated predominantly in a cephalad direction. In contrast, the helical blades exhibited a distinct migration in their axial direction. The Gamma3 hip screw design showed a significantly higher migration resistance compared with other screw and helical blade designs. The results demonstrate the ability of hip screws to significantly reduce axial migration and prevent cut-out under simulated walking loads. Further, the new multiplanar test method creates a physiological environment that can be used to optimize designs for intertrochanteric fracture fixation.

Anatomical plate configuration affects mechanical performance in distal humerus fractures.

BACKGROUND: Because of strong loads acting in the elbow joint, intraarticular fractures with a methaphyseal comminuted fracture site at the distal humerus demand a lot from the osteosynthetic care. Ambiguities arise concerning to the anatomic position of the implants and the resulting mechanic performance. The aim of this biomechanical study was to compare the performance of different anatomical plate configurations for fixation of comminuted distal humerus fractures within one system. METHODS: In an artificial bone model two perpendicular and one parallel plating configuration of a dedicated elbow plating system were compared with respect to system rigidity (flexion and extension) and dynamic median fatigue limit (extension). The flexion tests were conducted under 75° and the extension tests under 5°. Furthermore, the relative displacements were recorded. As a fracture model an AO C 2.3-fracture on an artificial bone (4th Gen. Sawbone) was simulated via double osteotomy in sagittal and transversal plane. FINDINGS: Large differences in mechanical performance were observed between flexion and extension loading modes. In extension the parallel configuration with lateral and medial plates achieved the highest bending stiffness and median fatigue limit. In flexion the highest bending stiffness was reached by the construct with a medial and a postero-lateral plate. Failure of the implant system predominantly occurred at the screw-bone interface or by fatigue of the plate around the screw holes. INTERPRETATION: All three plate configurations provided sufficient mechanical stability to allow early postoperative rehabilitation with a reduced loading protocol. Although the individual fracture pattern determines the choice of plate configuration, the parallel configuration with lateral and medial plates revealed biomechanical advantages in extension only.

Internal fixation of type-C distal femoral fractures in osteoporotic bone.

BACKGROUND: Fixation of distal femoral fractures remains a challenge, especially in osteoporotic bone. This study was performed to investigate the biomechanical stability of four different fixation devices for the treatment of comminuted distal femoral fractures in osteoporotic bone. METHODS: Four fixation devices were investigated biomechanically under torsional and axial loading. Three intramedullary nails, differing in the mechanism of distal locking (with two lateral-to-medial screws in one construct, one screw and one spiral blade in another construct, and four screws [two oblique and two lateral-to-medial with medial nuts] in the third), and one angular stable plate were used. All constructs were tested in an osteoporotic synthetic bone model of an AO/ASIF type 33-C2 fracture. Two nail constructs (the one-screw and spiral blade construct and the four-screw construct) were also compared under axial loading in eight pairs of fresh-frozen human cadaveric femora. RESULTS: The angular stable plate constructs had significantly higher torsional stiffness than the other constructs; the intramedullary nail with four-screw distal locking achieved nearly comparable results. Furthermore, the four-screw distal locking construct had the greatest torsional strength. Axial stiffness was also the highest for the four-screw distal locking device; the lowest values were achieved with the angular stable plate. The ranking of the constructs for axial cycles to failure was the four-screw locking construct, with the highest number of cycles, followed by the angular stable plate, the spiral blade construct, and two-screw fixation. The findings in the human cadaveric bone were comparable with those in the synthetic bone model. Failure modes under cyclic axial load were comparable for the synthetic and human bone models. CONCLUSIONS: The findings of this study support the concept that, for intramedullary nails, the kind of distal interlocking pattern affects the stabilization of distal femoral fractures. Four-screw distal locking provides the highest axial stability and nearly comparable torsional stability to that of the angular stable plate; the four-screw distal interlocking construct was found to have the best combined (torsional and axial) biomechanical stability.

Type of hip fracture determines load share in intramedullary osteosynthesis.

The choice of the appropriate implant continues to be critical for fixation of unstable hip fractures. Therefore, the goal of this study was to develop a numerical model to investigate the mechanical performance of hip fracture osteosynthesis. We hypothesized that decreasing fracture stability results in increasing load share of the implant and therefore higher stress within the implant. We also investigated the relationship of interfragmentary movement to the fracture stability. A finite element model was developed for a cephalomedullary nail within a synthetic femur and simulated a pertrochanteric fracture, a lateral neck fracture, and a subtrochanteric fracture. The femur was loaded with a hip force and was constrained physiologically. The FE model was validated by mechanical experiments. All three fractures resulted in similar values for stiffness (462-528 N/mm). The subtrochanteric fracture resulted in the highest local stress (665 MPa), and the pertrochanteric fracture resulted in a lower stress (621 MPa) with even lower values for the lateral neck fracture (480 MPa). Thus, intramedullary implants can stabilize unstable hip fractures with almost the same amount of stiffness as seen in stable fractures, but they have to bear a higher load share, resulting in higher stresses in the implant.

Influence of intramedullary nail diameter and locking mode on the stability of tibial shaft fracture fixation.

BACKGROUND: Fracture healing is affected by the type and the magnitude of movements at the fracture site. Mechanical conditions will be a function of the type of fracture management, the distance between the fracture fragments, and the loading of the fracture site. The hypothesis to be tested was that the use of a larger-diameter intramedullary nail, together with compressed interlocking, would enhance the primary stiffness and reduce fracture site movements, especially those engendered by shearing forces. MATERIALS AND METHODS: Six pairs of human tibiae were used to study the influence on fracture site stability of two different diameters (9 and 11 mm) of intramedullary nails, in tension/compression, torsional, four-point bending, and shear tests. The nails were used with two interlocking modes (static interlocking vs. dynamic compression). RESULTS: With static interlocking, the 11-mm-diameter nail provided significantly (30-59%) greater reduction of fracture site movement, as compared with the 9-mm-diameter nail. Using an 11-mm-diameter nail, the stiffness of the bone-implant construct was enhanced by between 20 and 50%. Dynamic compression allowed the interfragmentary movements at the fracture site to be further reduced by up to 79% and the system stiffness to be increased by up to 80%. CONCLUSION: On biomechanical grounds, the largest possible nail diameter should be used, with minimal reaming, so as to minimize fracture site movement. Compression after meticulous reduction should be considered in axially stable fractures.

Angle-stable and compressed angle-stable locking for tibiotalocalcaneal arthrodesis with retrograde intramedullary nails. Biomechanical evaluation.

BACKGROUND: Retrograde intramedullary nailing is an established procedure for tibiotalocalcaneal arthrodesis. The goal of this study was to evaluate the effects of angle-stable locking or compressed angle-stable locking on the initial stability of the nails and on the behavior of the constructs under cyclic loading conditions. METHODS: Tibiotalocalcaneal arthrodesis was performed in fifteen third-generation synthetic bones and twenty-four fresh-frozen cadaver legs with use of retrograde intramedullary nailing with three different locking modes: a Stryker nail with compressed angle-stable locking, a Stryker nail with angle-stable locking, and a statically locked Biomet nail. Analyses were performed of the initial stability of the specimens (range of motion) and the laxity of the constructs (neutral zone) in dorsiflexion/plantar flexion, varus/valgus, and external rotation/internal rotation. Cyclic testing up to 100,000 cycles was also performed. The range of motion and the neutral zone in dorsiflexion/plantar flexion at specific cycle increments were determined. RESULTS: In both bone models, the intramedullary nails with compressed angle-stable locking and those with angle-stable locking were significantly superior, in terms of a smaller range of motion and neutral zone, to the statically locked nails. The compressed angle-stable nails were superior to the angle-stable nails only in the synthetic bone model, in external/internal rotation. Cyclic testing showed the nails with angle-stable locking and those with compressed angle-stable locking to have greater stability in both models. In the synthetic bone model, compressed angle-stable locking was significantly better than angle-stable locking; in the cadaver bone model, there was no significant difference between these two locking modes. During cyclic testing, five statically locked nails in the cadaver bone model failed, whereas one nail with angle-stable locking and one with compressed angle-stable locking failed. CONCLUSIONS: Regardless of the bone model, the nails with angle-stable or compressed angle-stable locking had better initial stability and better stability following cycling than did the nails with static locking.

Biomechanical evaluation of primary stiffness of tibiotalocalcaneal fusion with intramedullary nails.

BACKGROUND: Intramedullary implants are being used with increasing frequency for tibiotalocalcaneal fusion (TTCF). Clinically, the question arises whether intramedullary (IM) nails should have a compression mode to enhance biomechanical stiffness and fusion-site compression. This biomechanical study compared the primary stability of TTCF constructs using compressed and uncompressed retrograde IM nails and a screw technique in a bone model. METHODS: For each technique, three composite bone models were used. The implants were a Biomet nail (static locking mode and compressed mode), a T2 femoral nail (compressed mode); a prototype IM nail 1 (PT1, compressed mode), a prototype IM nail 2 (PT2, dynamic locking mode and compressed mode), and a three-screw construct. The compressed contact surface of each construct was measured with pressure-sensitive film and expressed as percent of the available fusion-site area. Stiffness was tested in dorsiflexion and plantarflexion (D/P), varus and valgus (V/V), and internal rotation and external rotation (I/E) (20 load cycles per loading mode). RESULTS: Mean contact surfaces were 84.0 +/- 6.0% for the Biomet nail, 84.0 +/- 13.0% for the T2 nail, 70.0 +/- 7.2% for the PTI nail, and 83.5 +/- 5.5% for the compressed PT2 nail. The greatest primary stiffness in D/P was obtained with the compressed PT2, followed by the compressed Biomet nail. The dynamically locked PT2 produced the least primary stiffness. In V/V, PT1 had the (significantly) greatest primary stiffness, followed by the compressed PT2. The statically locked Biomet nail and the dynamically locked PT2 had the least primary stiffness in V/V. In I/E, the compressed PT2 had the greatest primary stiffness, followed by the PT1 and the T2 nails, which did not differ significantly from each other. The dynamically locked PT2 produced the least primary stiffness. The screw construct's contact surface and stiffness were intermediate. CONCLUSIONS: The IM nails with compression used for TTCF produced good contact surfaces and primary stiffness. They were significantly superior in these respects to the uncompressed nails and the screw construct. The large contact surfaces and great primary stiffness provided by the IM nails in a bone model may translate into improved union rates in patients who have TTCF.

Biomechanical evaluation of primary stiffness of tibiotalar arthrodesis with an intramedullary compression nail and four other fixation devices.

BACKGROUND: Many techniques of tibiotalar arthrodesis have been described. Screw fixation is widely used. At our center, intramedullary compression nailing has been successfully used for over 10 years. The question to be answered by this study was whether tibiotalar arthrodesis with a compressed intramedullary nail was superior, in terms of primary stiffness and fusion-site compression, to selected other techniques. METHODS: Plane fusion surfaces were machined in third-generation synthetic composite tibiae and the bodies of anatomically correct tali; fixation was with a compressed external fixator (cEF), an uncompressed interlocking nail (uIN), a compressed interlocking nail (cIN), and two different three-screw techniques (ST1 and ST2); three specimens per construct were tested. The compressed contact surface of each construct was measured with pressure-sensitive film and expressed as a percent of the available fusion-site area. Construct stiffness was tested in dorsiflexion/plantar flexion (D/P), varus/valgus (V/V), and internal rotation/external rotation (I/E), analyzing 20 cycles per loading mode. RESULTS: Compressed surface area: cIN 80% +/- 10.7; cEF 60% +/- 8.6; ST2 59% +/- 4.4; ST1 55% +/- 6.1; uIN no discernible compression. The greatest primary stiffness in D/P was obtained with the cIN (p < 0.001), followed by ST1. In V/V, ST1 and the cIN had the greatest primary stiffness; the two techniques did not differ significantly. Stabilization with the cEF was significantly better (p < 0.001) than with ST2. In I/E, the cIN produced the greatest primary stiffness (p < 0.001), followed by the two screw techniques, which did not differ significantly between themselves. The uIN had the least primary stiffness in all directions tested. CONCLUSIONS: In this biomechanical study, the cIN and ST1 were superior, in terms of primary stiffness in tibiotalar arthrodesis, to the other techniques tested. In D/P and I/E, the cIN construct was significantly stiffer than the ST1 construct, whose I/E rigidity might, however, be enhanced by a fourth, horizontal screw. CLINICAL RELEVANCE: Intramedullary compression nailing offers stable tibiotalar arthrodesis fixation with a large bony contact area and may enhance the likelihood of successful tibiotalar arthrodesis.