The effect of a collar on primary stability of standard and undersized cementless hip stems: a biomechanical study.

INTRODUCTION: Aseptic loosening and periprosthetic fractures are main reasons for revision after THA. Quite different from most other stem systems, Corail cementless hip stems show better survival rates than their cemented counterpart, which can possibly be explained by the use of a collar. The study aimed to investigate primary stability with standard and undersized hip stems both collared and collarless. MATERIALS AND METHODS: Primary stability of cementless, collared and collarless, femoral stems was measured in artificial bones using both undersized and standard size. After preconditioning, 3D micromotion was measured under cyclic loading at the bone-implant interface. RESULTS: The use of a collar resulted in higher micromotion within the same stem size but showed no statistically significant difference for both standard and undersized hip stems. The collared and collarless undersized stems showed no significant differences in 3D micromotion at the upper measuring positions compared to the standard stem size. Micromotion was significantly higher in the distal measuring positions, with and without collar, for the undersized stems (vs. standard collarless stem size). CONCLUSION: The key finding is that the collarless and collared Corail hip stems, within one stem size, showed no significant differences in primary stability. Undersized stems showed significantly higher micromotion in the distal area both with and without collar.

Motion preservation for open book injuries of the pubic symphysis -a biomechanical cadaver study.

INTRODUCTION: Open book injuries are challenging injuries that oftentimes require surgical treatment. Currently, treatment is performed with symphyseal plating requiring extensive surgery and entirely limiting physiological movement of the symphyseal joint, frequently resulting in implant failure. Therefore, we investigated the biomechanical properties of a minimally invasive tape suture construct (modified SpeedBridge™) as an alternative stabilization technique for the treatment of open book injuries in human cadaver pelvic rings. MATERIALS AND METHODS: The symphysis of 9 human cadaver pelvises was dissected and dilated to 3 cm creating an open book injury. Next, the two osteosynthesis methods (plating, modified SpeedBridge™) were applied. All specimens then underwent cyclic horizontal and vertical loading, simulating biomechanical forces while sitting, standing and walking. For statistical analysis, 3D dislocation (mm) was calculated. RESULTS: Total displacement (mm) of the pubic symphysis displayed the following means and standard deviations: native group 1.34 ± 0.62 mm, open book group 3.01 ± 1.26 mm, tape group 1.94 ± 0.59 mm and plate group 1.37 ± 0.41 mm. Comparison between native and open book (p = 0.029), open book and plate (p = 0.004), open book and tape (p = 0.031), as well as tape and plate group (p = 0.002) showed significant differences. No significant differences were found when comparing the native and tape (p = 0.059), as well as the native and plate (p = 0.999) group. CONCLUSION: While both osteosynthesis techniques sufficiently stabilized the injury, symphyseal plating displayed the highest rigidity. The modified SpeedBridge™ as a tape suture construct provided statistically sufficient biomechanical stability while maintaining symphyseal micro mobility, consequently allowing ligamental healing of the injured joint without iatrogenic arthrodesis.

Strain evaluation of axially loaded collateral ligaments: a comparison of digital image correlation and strain gauges.

The response of soft tissue to loading can be obtained by strain assessment. Typically, strain can be measured using electrical resistance with strain gauges (SG), or optical sensors based on the digital image correlation (DIC), among others. These sensor systems are already established in other areas of technology. However, sensors have a limited range of applications in medical technology due to various challenges in handling human soft materials. The aim of this study was to compare directly attached foil-type SG and 3D-DIC to determine the strain of axially loaded human ligament structures. Therefore, the medial (MCL) and lateral (LCL) collateral ligaments of 18 human knee joints underwent cyclic displacement-controlled loading at a rate of 20 mm/min in two test trials. In the first trial, strain was recorded with the 3D-DIC system and the reference strain of the testing machine. In the second trial, strain was additionally measured with a directly attached SG. The results of the strain measurement with the 3D-DIC system did not differ significantly from the reference strain in the first trial. The strains assessed in the second trial between reference and SG, as well as between reference and 3D-DIC showed significant differences. This suggests that using an optical system based on the DIC with a given unrestricted view is an effective method to measure the superficial strain of human ligaments. In contrast, directly attached SGs provide only qualitative comparable results. Therefore, their scope on human ligaments is limited to the evaluation of changes under different conditions.

Biomechanical stability of short versus long proximal femoral nails in osteoporotic subtrochanteric A3 reverse-oblique femoral fractures: a cadaveric study.

PURPOSE: Due to the demographic change towards an older society, osteoporosis-related proximal femur fractures are steadily increasing. Intramedullary nail osteosyntheses are available in different lengths, where the field of application overlaps. The aim of this study was to investigate whether subtrochanteric fractures can also be treated stably using a short femoral intramedullary nail in cadaveric bones. METHODS: A short PFNA and a long PFNA were implanted in both seven artificial bones and osteoporotic human specimens. A standardized AO 31-A3 (reverse-oblique) fracture was placed in the specimens with a lateral fracture spur 2 cm proximal to the distal locking screw (short PFNA) and embedded. The simulated iliotibial tract was preloaded to 50 N. The force was applied at 10 mm/min up to a force of 200-800 N (artificial bones) and 200-400 N (human specimens). The dislocation of the fracture gap, the axial bone stiffness of bone construct and the force curve of the tractus iliotibialis were measured. RESULTS: There is no difference in the use of a short versus long PFNA in terms of stiffness of the overall construct and only a slight increase in dislocation in the fracture gap results with short PFNA compared to a long intramedullary nail. CONCLUSION: In summary of the available literature, the present study supports the thesis that there is no clinical difference between long versus short nails in A3 femur fractures. Furthermore, the present study defines a safe biomechanical range of fracture extension above the locking screw of the short intramedullary nail. LEVEL OF EVIDENCE: III.

Tape suture constructs for instabilities of the pubic symphysis: is the idea of motion preservation a suitable treatment option? A cadaver study.

INTRODUCTION: Current gold standard for the treatment of symphyseal disruptions includes anterior plating, almost entirely prohibiting symphyseal mobility and resulting in an iatrogenic arthrodesis followed by high rates of implant failure. Minimally invasive tape suture constructs have been found to maintain the micro mobility of ligamentous injuries, yet still providing sufficient biomechanical stability. Recently, this technique has been primarily investigated for symphyseal disruptions on synthetic pelvic models. Therefore, the aim of this study was to examine the feasibility of this novel flexible osteosynthesis on cadaveric pelvic models based on the following hypothesis: tape suture constructs ensure sufficient biomechanical stability without inhibiting micro mobility of the pubic symphysis for the treatment of symphyseal disruptions and maintain stability during long-term loading. MATERIALS AND METHODS: 9 cadaveric anterior pelvic rings were used in this study and a symphyseal disruption was created in every specimen. The specimens were then exposed to short- and long-term vertical and horizontal cyclic loading after treatment with a tape suture construct in criss-cross technique. The mean maximum displacement (mm) during cyclic loading and the corresponding stiffness (N/mm) were measured and compared. RESULTS: Regarding both displacement (mm) and corresponding stiffness (N/mm), the tape sutures displayed a significant difference between short- and long-term loading for cranial and caudal vertical loading (p < 0.01) but differences remained non-significant for horizontal loading (p > 0.05). No tape suture suffered from implant failure during long-term loading. CONCLUSIONS: The tape suture construct displayed sufficient biomechanical stability without exceeding the physiological mobility of 2 mm of the pubic symphysis; however, also maintained the desired micro mobility of the affected joint necessary to prevent an iatrogenic arthrodesis. Further, all tape sutures maintained stability throughout long-term loading.

Improving stability of atlantoaxial fusion: a biomechanical study.

PURPOSE: The incidence of atlanto-axial injuries is continuously increasing and often requires surgical treatment. Recently, Harati developed a new procedure combining polyaxial transarticular screws with polyaxial atlas massae lateralis screws via a rod system with promising clinical results, yet biomechanical data is lacking. This biomechanical study consequently aims to evaluate the properties of the Harati technique. METHODS: Two groups, each consisting of 7 cervical vertebral segments (C1/2), were formed and provided with a dens axis type 2 fracture according to Alonzo. One group was treated with the Harms and the other with the Harati technique. The specimen was loaded via a lever arm to simulate extension, flexion, lateral flexion and rotation. For statistical analysis, dislocation (°) was measured and compared. RESULTS: For extension and flexion, the Harati technique displayed a mean dislocation of 4.12° ± 2.36° and the Harms technique of 8.48° ± 1.49° (p < 0.01). For lateral flexion, the dislocation was 0.57° ± 0.30° for the Harati and 1.19° ± 0.25° for the Harms group (p < 0.01). The mean dislocation for rotation was 1.09° ± 0.48° for the Harati and 2.10° ± 0.31° for the Harms group (p < 0.01). No implant failure occurred. CONCLUSION: This study found a significant increase in biomechanical stability of the Harati technique when compared to the technique by Harms et al. Consequently, this novel technique can be regarded as a promising alternative for the treatment of atlanto-axial instabilities.

Novel minimally invasive tape suture osteosynthesis for instabilities of the pubic symphysis: a biomechanical study.

INTRODUCTION: Open book fractures are challenging injuries oftentimes requiring surgical treatment. The current treatment of choice is symphyseal plating, which requires extensive surgery and entirely limits physiological movement of the symphyseal joint, frequently resulting in implant failure. Therefore, we investigated the biomechanical properties of a semi-rigid implant (modified SpeedBridge™) as a minimally invasive tape suture construct for the treatment of open book fractures and evaluated the superiority of two techniques of implementation: criss-cross vs. triangle technique. MATERIALS AND METHODS: Nine synthetic symphyseal joints were dissected creating an open book fracture. The different osteosynthesis methods (plating, modified SpeedBridge™ in criss-cross/triangle technique) were then applied. All constructs underwent horizontal and vertical loading, simulating biomechanical forces while sitting, standing and walking. For statistical analysis, dislocation (mm) and stiffness (N/mm) were calculated. RESULTS: Symphyseal plating for the treatment of open book fractures proved to be a rigid osteosynthesis significantly limiting the physiological mobility of the symphyseal joint (dislocation: 0.08 ± 0.01 mm) compared to the tape sutures (dislocation: triangle technique 0.27 ± 0.07 mm, criss-cross technique 0.23 ± 0.05 mm) regarding horizontal tension (p < 0.01). Both modified SpeedBridge™ techniques showed sufficient biomechanical stability without one being superior to the other (p > 0.05 in all directions). Considering vertical loading, no statistical difference was found between all osteosynthesis methods (caudal: p = 0.41; cranial: p = 0.61). CONCLUSIONS: Symphyseal plating proved to be the osteosynthesis method with the highest rigidity. The modified SpeedBridge™ as a semi-rigid suture construct provided statistically sufficient biomechanical stability while maintaining a minimum of symphyseal movement, consequently allowing ligamental healing of the injured joint without iatrogenic arthrodesis. Furthermore, both the criss-cross and the triangle technique displayed significant biomechanical stability without one method being superior.

Anatomical and biomechanical evaluation of the lateral calcaneo-cuboid and bifurcate ligaments.

BACKGROUND: This study aims to analyze the ligaments of the dorso-lateral calcaneo-cuboid joint and to assess the biomechanical relevance of the bifurcate ligament. METHODS: 16 specimens were analyzed for their ligamentous anatomy of the dorso-lateral calcaneo-cuboid joint and side-alternating assigned to two groups with varying ligamentous dissection order. The Chopart joint was stressed in plantar, medial, and lateral direction measuring the displacement by an 3D motion tracker for every dissection step. RESULTS: 37.5% of specimens had all ligaments (lateral calcaneo-cuboid, dorsal calcaneo-cuboid, bifurcate calcaneo-cuboid, bifurcate calcaneo-navicular), 37.5% were lacking bifurcate´s calcaneo-cuboid-portion, and 25% presented without dorsal calcaneo-cuboid. Biomechanical testing revealed no significant displacement within the calcaneo-cuboid or talo-navicular joint for any stressed state except for axial compression with dissected dorsal talo-navicular joint capsule in Group 2. CONCLUSION: Broad morphological variability and missing significant displacement regardless of its integrity, make the bifurcate ligament appear of limited biomechanical relevance.

Varus or valgus positioning of the tibial component of a unicompartmental fixed-bearing knee arthroplasty does not increase wear.

PURPOSE: Higher revision rates were shown in varus- or valgus-positioned tibias in unicompartmental knee arthroplasty (UKA), but more than 15% of UKA prostheses are implanted with more than 5° of varus or valgus. This study aimed to analyze the wear rate in UKA when implanting the tibial component in either varus or valgus position versus a neutral placement at 90° to the tibial anatomical axis. The study hypothesized that a 5° varus or valgus positioning of the tibial plateau will generate less wear compared to a neutral alignment. METHODS: Wear was experimentally analyzed on a medial anatomical fixed-bearing unicompartmental knee prosthesis (Univation, Aesculap, Germany) in vitro with a customized, four-station, servohydraulic knee wear simulator, reproducing the walking cycle. The forces, loading and range of motion were applied as specified in the ISO 14243-1:2002, 5 million cycles were analyzed. The tibial components of the medial prostheses were inserted in a neutral position, with 5° varus, and 5° valgus (n = 3, each group). RESULTS: The wear rate decreased significantly with a 5° varus positioning (6.30 ± 1.38 mg/million cycles) and a 5° valgus positioning (4.96 ± 2.47 mg/million cycles) compared to the neutral position (12.16 ± 1.26 mg/million cycles) (p < 0.01 for the varus and the valgus position). The wear area on the inlay was slightly reduced in the varus and valgus group. CONCLUSION: A varus or valgus "malpositioning" up to 5° will not lead to an increased wear. Wear was even less because of the reduced articulating contact area between the inlay and the femur. A slight varus positioning of the tibial component (parallel to the anatomical joint line) positioning can be advocated from a point of wear. LEVEL OF EVIDENCE: Experimental study.

TKA design-integrated trochlea groove rotation reduces patellofemoral pressure.

PURPOSE: Total knee arthroplasty (TKA) leaves 11-25% of the patients unsatisfied, and patellofemoral joint pain is one cause. This study aimed to compare the differences between kinematics and load transfer in the same knee with axial internal/external rotation of the femoral component (CoRo) versus a separate axial internal/external trochlear groove rotation (TrRo) which is included in the TKA trochlea design. METHODS: A validated weight-bearing finite element model with modifications of the TKA axial femoral component rotation (CoRo) and a modified trochlear rotation (TrRo) was calculated and analysed. RESULTS: Compared to the neutrally implanted TKA at 105° of flexion, a 6° external rotation of the trochlear groove reduced the retropatellar stress by 7%, whereas a 3° internal trochlear groove rotation increased the retropatellar stress by 7%. With femoral component rotation, the tibia inlay stress of 6.7 MPa at 60° of flexion was two times higher both with a 3° internal component rotation and a 6° external rotation. CONCLUSION: These results demonstrate in the tested TKA design that a trochlear groove rotation can reduce retropatellar stress. Additionally, during the TKA operation, the surgeon should be aware of the significant influence of axial femoral component rotation on mechanical inlay stress during flexion and of the fact that even small changes in the patellofemoral joint may influence the tibiofemoral joint. These results support that an external rotation of the femoral component should be preferred in TKA to avoid anterior knee pain. Furthermore, new developed TKA designs should integrate an externally rotated trochlea groove.

Influence of undersized cementless hip stems on primary stability and strain distribution.

INTRODUCTION: Undersizing of cementless hip stems is a risk factor for aseptic loosening and early subsidence. The purpose of this study was to evaluate the effects of undersized stems and determine whether a biomechanical study can predict the clinical results. MATERIALS AND METHODS: Three consecutive sizes of a clinically proven stem (CLS Spotorno) were implanted into six composite femora (size large, Sawbones®), respectively. According to the Canal Fill Index (CFI), two stems (size 11.25 and 12.5) were undersized (CFI < 80%) and one stem (size 13.75) had an appropriate size (CFI > 80%). The primary stability was evaluated by measurement of 3-dimensional (3D)-micromotions under physiological adapted load and surface strains were recorded before and after implantation to detect stress-shielding processes. RESULTS: Both undersized stems revealed significantly higher micromotions in all regions compared to the appropriate stem. The highest micromotions were registered at the distal tip of the three stem sizes. The changes in surface strain did not show a significant difference between the three stem sizes, but the highest strain reduction was observed proximally indicating a tendency for stress shielding. CONCLUSIONS: This study confirms the clinical assumption that undersized stem result in a significantly reduced primary stability. Furthermore, in vitro studies allow to determine the effects of undersizing and stress shielding processes.

[Implant anchorage: In vivo and in vitro analyses : Clusters for implant anchorage and safety]. / Implantatverankerung: In-vivo- und In-vitro-Analysen : Cluster Implantatverankerung und -sicherheit.

Aseptic implant loosening is the primary cause of revisions in arthroplasty. Various in vitro and in vivo methods are available for assessing implant fixation and stability. The aim of the Musculoskeletal Biomechanics Research Network (MSB-NET) is to continuously improve or develop these methods. In vitro analyses are often conducted using static and dynamic ISO and ASTM standards, while RSA, DXA, and EBRA analyses are established in vivo methods for evaluating implant fixation. Primary stability analyses, as well as acoustical methods, provide additional opportunities to detect loosening early and precisely evaluate implant stability. The cluster serves as a link between basic research, clinical practice, and end users to promote in vitro and in vivo methods to improve implant safety.

Biomechanical comparison of screw vs. cerclage refixation in orthogeriatric lesser trochanteric fractures: a cadaveric study.

PURPOSE: Osteoporosis-related proximal femur fractures continue to increase significantly due to demographic change. This study was designed to evaluate the biomechanical stability of two different fixation methods (cerclage vs. screw) for refixation of a trochanter minor fragment in the pertrochanteric fractures in cadaveric bones. METHODS: Artificial bones (n = 14) and human bones (n = 16) were treated with a DHS and the trochanter minor fragment was reduced by cerclage wiring or direct screw fixation. After preloading the simulated iliopsoas with 10 N, a tensile test was performed, ending with either a 70% loss of strength or avulsion of the fragment. The mean values of the avulsion force and the surface strain were recorded. RESULTS: All tensile tests showed no significant differences between refixation using a direct screw or wire cerclage, for both artificial bones and human specimens. Absolute values showed higher avulsion forces after direct screw fixation than refixation with a wire cerclage. The surface tension of specimens treated with direct screw fixation was lower than that of specimens treated with wire cerclage. An opposite effect was seen in artificial bones. Both effects were not statistically significant. CONCLUSION: Based on the equal stability after lag screw placement compared to cerclage wiring, we promote the placement of a lag screw into the lesser trochanter fragment in pertrochanteric femur fractures when using a dynamic hip screw. LEVEL OF EVIDENCE: Level III.

Novel Tape Suture Osteosynthesis for Hyperextension Injuries of the Subaxial Cervical Spine: A Biomechanical Study.

BACKGROUND/AIM: Anterior tension band injuries are usually the result of high impact hyperextension trauma. Current surgical treatment includes anterior cervical discectomy and fusion bearing the risk of soft tissue irritation, degeneration of adjacent cervical segments, implant failure or iatrogenic spondylodesis. This study examined the biomechanical properties of tape suture constructs reenforcing ligamental stability for the treatment of Association of Osteosynthesis (AO) type B3 injuries compared to anterior fusion. MATERIALS AND METHODS: After creation of an AO type B3 injury in synthetic cervical segments (C5/6, Sawbone®), seven segments were treated with anterior fusion and seven with a tape suture construct, similar to the SpeedBridge™ (Arthrex®). Biomechanical testing was performed, simulating extension, flexion, lateral bending, and rotation. Dislocation (°) and corresponding force (N) were measured and compared. RESULTS: Anterior fusion displayed a mean range of extension, lateral bending, and rotation of 3.60° (SD 1.87°), 2.28° (SD 1.55°), and 2.81° (SD 0.78°), respectively. The tape suture showed a mean range of extension, lateral bending, and rotation of 4.24° (SD 0.81°) (p=0.146), 5.44° (SD 1.56°) (p=0.013), and 5.29° (SD 1.44°) (p<0.01), respectively. No specimen suffered from implant failure. CONCLUSION: The tape suture construct provides sufficient biomechanical stability for the treatment of AO type B3 injuries compared to anterior fusion. Regarding cervical extension, whose limitation is crucial for ligamental healing, the tape suture shows no significant inferiority. Yet, the tape suture approaches physiological mobility in the planes not affected by the injury. Consequently, the tape suture is a promising alternative preventing an iatrogenic spondylodesis.

Micro-Structural and Biomechanical Evaluation of Bioresorbable and Conventional Bone Cements for Augmentation of the Proximal Femoral Nail.

Osteoporotic proximal femur fractures are on the rise due to demographic change. The most dominant surgical treatment option for per/subtrochanteric fractures is cephalomedullary nailing. As it has been shown to increase primary stability, cement augmentation has become increasingly popular in the treatment of osteoporotic per/subtrochanteric femur fractures. The ultimate goal is to achieve stable osteosynthesis, allowing for rapid full weight-bearing to reduce possible postoperative complications. In recent years, bioresorbable bone cements have been developed and are now mainly used to fill bone voids. The aim of this study was to evaluate the biomechanical stability as well as the micro-structural behaviour of bioresorbable bone cements compared to conventional polymethylmethacrylate (PMMA)-cements in a subtrochanteric femur fracture model. Biomechanical as well as micro-computed tomography morphology analysis revealed no significant differences in both bone cements, as they showed equal mechanical stability and tight interdigitation into the spongious bone of the femoral head. Given the positive risk/benefit ratio for bioresorbable bone cements, their utilisation should be evaluated in future clinical studies, making them a promising alternative to PMMA-bone cements.

Influence of Treadmill Design on Gait: Does Treadmill Size Affect Muscle Activation Amplitude? A Musculoskeletal Calculation With Individualized Input Parameters of Gait Analysis.

With increasing age, gait changes often occur, leading to mobility problems and thus a higher risk of falling. Interest in training at home or at retirement homes has led to the development of "mobile treadmills." A difference in treadmill surface length may influence walking parameters (i.e., step length) and therefore may affect muscle activation. This led to the question: Does the treadmill size affect the muscle activation, i.e., with the length of the walking surface. The study aimed to investigate the influence of treadmill size, i.e., length of the walking surface, on gait pattern and to determine differences in the amplitude of muscle activation using a participant-specific musculoskeletal model (AnyBody Technology A/S, Aalborg, Denmark). For a prospective, randomized study gait parameters were collected from 47 healthy participants (aged 50.19 ± 20.58 years) while walking on two different treadmills, a small mobile treadmill (walking surface length 100 cm) and a conventional treadmill (walking surface length 150 cm), at their preferred speed, 2 km/h, and 4 km/h. Muscle activation amplitude patterns were similar between treadmills (M. gastrocnemius medialis: rmean = 0.94, M. gastrocnemius lateralis: rmean = 0.92, M. gluteus medius rmean = 0.90, M. gluteus minimus rmean = 0.94). However, the gait analysis showed a decreased preferred velocity (p < 0.001, z = 4.54), reduced stride length (preferred velocity: p = 0.03, z = -2.17; 2 km/h: p = 0.36, z = 2.10; 4 km/h: p = 0.006, z = 2.76), shorter stride time (2 km/h: p < 0.001, z = 4.65; 4 km/h: p < 0.001, z = 4.15), and higher cadence (2 km/h: p < 0.001, z = -4.20; 4 km/h: p = 0.029, z = -2.18) on the mobile treadmill than on the conventional treadmill. Our observations suggest that the treadmill design (e.g., a 50 cm difference in walking surface length) may not influence muscle activity amplitude during walking. However, the design of the treadmill may influence gait characteristics (e.g., stride length, cadence) of walking.

Influence of different anteversion alignments of a cementless hip stem on primary stability and strain distribution.

BACKGROUND: Stem anteversion in total hip arthroplasty is well known to have a high impact on dislocation, but empirical data regarding the clinical and biomechanical influence is lacking. Therefore, we evaluated the impact of different anteversion alignments on the primary stability and strain distribution of a cementless stem. METHODS: The cementless CLS Spotorno stem was implanted in 3 different groups (each group n = 6, total n = 21) with different anteversion alignments: reference anteversion (8°), +15° torsion in anteversion (+23°), -15° torsion in retroversion (-7°) using composite femurs (Sawbones). Primary stability was determined by 3-dimensional micromotions using a dynamic loading procedure simulating walking on level ground. Additionally, surface strains were registered before and after stem insertion in the 3 different groups, using one composite femur for each group (total n = 3). FINDINGS: The micromotion measurements did not show a significant difference between the 3 evaluated alignments. Moreover, determination of the strain distribution did also not reveal an obvious difference. INTERPRETATION: This biomechanical study simulating walking on level ground indicates that there is no considerable influence of stem ante-/retroversion variation (±15°) on the initial stability and strain distribution when evaluating the cementless CLS Spotorno in composite femora.

Varus malalignment of cementless hip stems provides sufficient primary stability but highly increases distal strain distribution.

BACKGROUND: Varus position of cementless stems is a common malalignment in total hip arthroplasty. Clinical studies have reported a low rate of aseptic loosening but an increased risk for thigh pain. This in vitro study aimed to evaluate these clinical observations from a biomechanical perspective. METHODS: A conventional cementless stem (CLS Spotorno) was implanted in a regular, straight (size 13.75) as well as in a varus position (size 11.25) in 6 composite femora (Sawbones), respectively. Primary stability was assessed by recording 3-dimensional micromotions under dynamic load bearing conditions and stress shielding was evaluated by registering the surface strain before and after stem insertion. FINDINGS: Primary stability for stems in varus malposition revealed significantly lower micromotions (p < 0.05) for most regions compared to stems in neutral position. The greatest difference was observed at the tip of the stem where the straight aligned implants exceeded the critical upper limit for osseous integration of 150 µm. The surface strains for the varus aligned stems revealed a higher load transmission to the femur, resulting in a clearly altered strain distribution. INTERPRETATION: This biomechanical study confirms the clinical findings of a good primary stability of cementless stems in a varus malposition, but impressively demonstrates the altered load transmission with the risk for postoperative thigh pain.

Influence of different sizes of composite femora on the biomechanical behavior of cementless hip prosthesis.

BACKGROUND: For the biomechanical evaluation of cementless stems different sizes of composite femurs have been used in the literature. However, the impact of different specimen sizes on test results is unknown. METHODS: To determine the potential effect of femur size the biomechanical properties of a conventional stem (CLS Spotorno) were examined in 3 different sizes (small, medium and large composite Sawbones®). Primary stability was tested under physiologically adapted dynamic loading conditions measuring 3-dimensional micromotions. For the small composite femur the dynamic load needed to be adapted since fractures occurred when reaching 1700N. Additionally, surface strain distribution was recorded before and after implantation to draw conclusions about the tendency for stress shielding. FINDINGS: All tested sizes revealed similar micromotions only reaching a significant different level at one measurement point. The highest micromotions were observed at the tip of the stems exceeding the limit for osseous integration of 150µm. Regarding strain distribution the highest strain reduction after implantation was registered in all sizes at the level of the lesser trochanter. INTERPRETATION: Specimen size seems to be a minor influence factor for biomechanical evaluation of cementless stems. However, the small composite femur is less suitable for biomechanical testing since this size failed under physiological adapted loads. For the CLS Spotorno osseous integration is unlikely at the tip of the stem and the tendency for stress shielding is the highest at the level of the lesser trochanter.