Biomechanics of subtrochanteric fracture fixation using short cephalomedullary nails: A finite element analysis.

A finite element analysis was performed to evaluate the stresses around nails and cortical bones in subtrochanteric (ST) fracture models fixed using short cephalomedullary nails (CMNs). A total 96 finite element models (FEMs) were simulated on a transverse ST fracture at eight levels with three different fracture gaps and two different distal locking screw configurations in both normal and osteoporotic bone. All FEMs were fixed using CMNs 200 mm in length. Two distal locking screws showed a wider safe range than 1 distal screw in both normal and osteoporotic bone at fracture gaps ≤ 3 mm. In normal bone FEMs fixed even with two distal locking screws, peak von Mises stresses (PVMSs) in cortical bone and nail constructs reached or exceeded 90% of the yield strength at fracture levels 50 mm and 0 and 50 mm, respectively, at all fracture gaps. In osteoporotic bone FEMs, PVMSs in cortical bone and nail constructs reached or exceeded 90% of the yield strength at fracture levels 50 mm and 0 and 50 mm, respectively, at a 1-mm fracture gap. However, at fracture gaps ≥ 2 mm, PVMSs in cortical bone reached or exceeded 90% of the yield strength at fracture levels ≥ 35 mm. PVMSs in nail showed the same results as 1-mm fracture gaps. PVMSs increased and safe range reduced, as the fracture gap increased. Short CMNs (200 mm in length) with two distal screws may be considered suitable for the fixation of ST transverse fractures at fracture levels 10 to 40 mm below the lesser trochanter in normal bone and 10 to 30 mm in osteoporotic bone, respectively, under the assumptions of anatomical reduction at fracture gap ≤ 3 mm. However, the fracture gap should be shortened to the minimum to reduce the risk of refracture and fixation failure, especially in osteoporotic fractures.

Effect of stem position and length on bone-stem constructs after cementless hip arthroplasty.

AIMS: There are concerns regarding initial stability and early periprosthetic fractures in cementless hip arthroplasty using short stems. This study aimed to investigate stress on the cortical bone around the stem and micromotions between the stem and cortical bone according to femoral stem length and positioning. METHODS: In total, 12 femoral finite element models (FEMs) were constructed and tested in walking and stair-climbing. Femoral stems of three different lengths and two different positions were simulated, assuming press-fit fixation within each FEM. Stress on the cortical bone and micromotions between the stem and bone were measured in each condition. RESULTS: Stress concentration was observed on the medial and lateral interfaces between the cortical bone and stem. With neutral stem insertion, mean stress over a region of interest was greater at the medial than lateral interface regardless of stem length, which increased as the stem shortened. Mean stress increased in the varus-inserted stems compared to the stems inserted neutrally, especially at the lateral interface in contact with the stem tip. The maximum stress was observed at the lateral interface in a varus-inserted short stem. All mean stresses were greater in stair-climbing condition than walking. Each micromotion was also greater in shorter stems and varus-inserted stems, and in stair-climbing condition. CONCLUSION: The stem should be inserted neutrally and stair-climbing movement should be avoided in the early postoperative period, in order to preserve early stability and reduce the possibility of thigh pain, especially when using a shorter stem. Cite this article: Bone Joint Res 2021;10(4):250-258.

Biomechanical comparison of the angle of inserted screws and the length of anterior cervical plate systems with allograft spacers.

BACKGROUND: Comparative studies of the biomechanical effects of plates of varying lengths and different screw insertion angles on allograft spacers are lacking. METHODS: Finite element model analysis of a previously validated, three-dimensional, intact cervical spinal segment model of C3-6 was conducted in the present study. On the C5-6 segment, anterior discectomy and fusion were performed using allograft spacers and different combinations of anterior plates and screws. The biomechanical characteristics of combinations of short, medium, and maximal length plates with screw insertion angles of 0°, 8°, 16°, and 32° were analyzed. FINDINGS: In flexion and extension, the risk of allograft spacer subsidence decreased as screw angles increased. Short plates with a screw insertion angle of 32° posed the lowest subsidence risk, similar to medium length plates with a screw insertion angle of 16°, in all motion conditions. The risk of bone yielding increased as plate length increased, but decreased as the screw insertion angle increased. INTERPRETATION: Short plates with a large screw insertion angle (32°) showed the highest mechanical stability and load sharing of allograft spacers and the lowest risk of screw loosening. Accordingly, we recommend the use of a short plate and large screw insertion angle for anterior cervical discectomy and fusion.

Biomechanical comparison of cervical discectomy/fusion model using allograft spacers between anterior and posterior fixation methods (lateral mass and pedicle screw).

BACKGROUND: The purpose of this study is to investigate effects of different fixation methods on the physical stress on allospacers, endplate-vertebral body, and implants using finite element model analyses. METHODS: Stress distribution and subsidence risk according to the fixation methods under the condition of hybrid motion control were analyzed. The detailed finite element model of a previously validated, three-dimensional, intact cervical spinal segment model, with C5-C6 segmental fusion using allospacer, was used to evaluate the biomechanical characteristics of different fixation combinations, such as anterior plate/screws, lateral mass screw, and posterior pedicle screw. FINDINGS: The load sharing on allospacers increased in extension in order of posterior pedicle screws (21.4%), lateral mass screws (31.5%), and anterior plate/screws (56.6%). lateral mass screw demonstrated the highest load sharing (68.1%) on the allospacer in flexion. The Peak von Mises stress of the allospacer was the lowest in flexion and axial rotation but the highest in extension with anterior plate/screws. Allospacer subsidence risk was the lowest in extension, lateral bending, and axial rotation with posterior pedicle screws but the lowest in flexion with anterior plate/screws. The bone-screw loosening risk was the lowest in all modes with posterior pedicle screws but the highest with anterior plate/screws. INTERPRETATION: Posterior pedicle screws demonstrated the best mechanical stability of allospacer failure-subsidence and the lowest risk of screw loosening. Different motion restrictions depending on the fixation method should be considered for implant and allospacer safety.

Effect of fracture levels on the strength of bone-implant constructs in subtrochanteric fracture models fixed using short cephalomedullary nails: A finite element analysis.

OBJECTIVES: This study was conducted to investigate the stress around nails and cortical bones in subtrochanteric (ST) fractures fixed using short cephalomedullary nails (CMNs) in finite element models (FEMs) and to determine the appropriate short CMN type for different fracture levels. METHODS: The following three types of short CMNs were used: type A, which is 170 mm in length and has 1 distal locking screw; type B, 200 mm in length and 1 distal screw; and type C, 200 mm in length and 2 distal screws. A total of 24 FEMs were tested on a transverse ST fracture at 8 levels [0, 10, 20, 25, 30, 35, 40 and 50 mm below the lower margin of lesser trochanter (LT)], and were fixed using 3 different CMN types. Finite element analysis was then performed to evaluate the stress around the cortical bones and the CMNs under the assumption of anatomical reduction and fracture gap of 1 mm. RESULTS: Peak von Mises stress (PVMS) was greatest on the cortical bone around the distal screw hole and was greater than the yield strength at fracture levels ≥ 35 mm below the LT in FEMs fixed with type A and B. In contrast, FEMs fixed with type C showed PVMS less than the yield strength at all fracture levels. The PVMS within the implant was greater than the yield strength at the junction of the nail with the distal screw and distal screw itself at fracture levels ≥ 35 mm below the LT in FEMs fixed using type A. Conversely, in FEMs fixed using type B and C, all PVMSs within the implant were less than the yield strength, regardless of the fracture level. CONCLUSION: Short CMNs 170 or 200 mm in length with 1 distal screw may be used in a limited manner in high ST transverse fractures under the assumptions of anatomical reduction and fracture gap ≤ 1 mm. Meanwhile, short CMN 200 mm in length with 2 distal screws may be an available treatment option in most of ST transverse fractures regardless of the fracture level under the same set of assumptions.