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A modified and enhanced test setup for biomechanical investigations of the hindfoot, for example in tibiotalocalcaneal arthrodesis
- Julia Evers†1,
- Martin Schulze†1,
- Dominic Gehweiler1,
- Martin Lakemeier1,
- Michael J. Raschke1,
- Dirk Wähnert†1 and
- Sabine Ochman†1Email author
© Evers et al. 2016
Received: 3 May 2015
Accepted: 28 June 2016
Published: 29 July 2016
Tibiotalocalcaneal arthrodesis (TTCA) using intramedullary nails is a salvage procedure for many diseases in the ankle and subtalar joint. Despite “newly described intramedullary nails” with specific anatomical shapes there still remain major complications regarding this procedure. The following study presents a modified biomechanical test setup for investigations of the hindfoot.
Nine fresh-frozen specimens from below the human knee were anaysed using the Hindfoot Arthrodesis Nail (Synthes) instrument. Quasi-static biomechanical testing was performed for internal/external rotation, varus/valgus and dorsal/plantar flexion using a modified established setup (physiological load entrance point, sledge at lever arm to apply pure moments). Additionally, a 3D optical measurement system was added to allow determination of interbony movements.
The mean torsional range of motion (ROM) calculated from the actuator data of a material testing machine was 10.12° (SD 0.6) compared to 10° (SD 2.83) as measured with the Optotrak® system (between tibia and calcaneus). The Optotrak showed 40 % more rotation in the talocrural joint.
Mean varus/valgus ROM from the material testing flexion machine was seen to be 5.65° (SD 1.84) in comparison to 2.82° (SD 0.46) measured with the Optotrak. The subtalar joint showed a 70 % higher movement when compared to the talocrural joint.
Mean ROM in the flexion test was 5.3° (SD 1.45) for the material testing machine and 2.1° (SD 0.39) for the Optotrak. The movement in the talocrural joint was 3 times higher compared to the subtalar joint.
The modified test setup presented here for the hindfoot allows a physiological biomechanical loading. Moreover, a detailed characterisation of the bone-implant constructs is possible.
Tibiotalocalcaneal arthrodesis (TTCA) using an intramedullary nail is a salvage procedure for many diseases involving the talocrural and subtalar joint. Since the first report of arthrodesis with retrograde femoral nails, new generations of intramedullary nails have been designed in regard to the specific anatomy of the ankle.
It is common sense, that instability, especially torsional and shear stress, has negative impact on bone healing. In case of the hindfoot arthrodesis three bones and two joints are involved, which can cause instability und thus promote a non-union. Therefore, comparing overall construct stability is insufficient, attentions has to be directed to the stability of the talocrural and subtalar joint separately.
Actual biomechanical setups are able to compare the complete bone-implant- constructs only, without offering any differentiation of talocrural and subtalar joint movement. An established biomechanical method [9–11] was therefore modified and enhanced to address this disadvantage. Furthermore, the load entry point on the construct was adapted to mimic more physiological conditions. Additionally, a 3D motion tracking system was added to evaluate movements between all joint sections (tibia, talus, calcaneus) individually.
Nine fresh-frozen human specimens from below the knee taken from our Institute of Anatomy were used for this study (five female, four male donors). Median age of the donors was 85.3 years (range: 77–95 years), four right and five left tibiae were available. The specimens were stored at -18C and thawed at room temperature for 24 h before testing.
For screening of pre-existing bony pathologies and implant size estimation, conventional radiographs in two planes (antero-posterior and lateral) were obtained. Additional bone mineral density of the cancellous bone in the calcaneus was measured by quantitative computed tomography (Somatom Definition, Siemens, Erlangen, Germany).
The specimens were prepared by removing all soft-tissue except for the stabilizing ligaments at the ankle such as the distal syndesmotic complex, the interosseous membrane, collateral ligaments and the capsule of the ankle and subtalar joints. The joint surfaces of the ankle and subtalar joints were left intact following former biomechanical studies [9, 12]. The forefoot was amputated at the Chopart’s joint. In contrast to the study of Klos et al. the fibula was not resected, in order to prevent additional instability . Tibia and fibula were transected 30 cm above the ankle joint.
An experienced orthopaedic surgeon, following the manufacturers’ guidelines and using the adequate tools under c-arm control, performed implantation.
In contrast to the intraoperative procedure, the joint cartilage was not removed to ensure a standardized procedure.
The test setup used in this study was based on the publications of Mückley et al. and Klos et al. [9, 10]. They used an upside down positioning of the specimen with force application via a 80 mm lever arm using a connecting rod for varus/valgus and flexion testing, for torsional testing force was applied directly to the embedding .
For mechanical testing a material testing machine (Instron 8874, Instron, High Wycombe, Bucks, United Kingdom) was used. We modified the test setup in two main aspects:
Before embedding, all screw holes and the subtalar joint were covered using modelling clay to prevent any influence on the biomechanical properties, especially artificial fusion of the subtalar joint due to misplaced bone cement and infiltration of the cement into the medullary canal. First the calcaneus was embedded using Technovit (Technovit 3040 Heraeus Kulzer GmbH, Wernheim, Germany) and a special casing to allow a radiological view on the calcaneus. Afterwards, the tibia and fibula were embedded, verifying the adjustment and positioning by a cross-line laser. To exclude shear stress, the proximal embedding was performed on the testing machine.
quasi-static tests (20 cycles) in
o dorsi-/plantarflexion (±62.5 N equal to ±5 Nm) at 0.5 Hz
o varus/valgus (±62.5 N equal to ±5 Nm) at 0.5 Hz
o torsion in external-/internal rotation (±5 Nm) at 1°/sec
After 10 cycles of loading (to exclude setting behaviour) 10 cycles of mechanical loading have been applied to the specimens. For these 10 cycles the range of motion and neutral zone has been determined for every specimen, for further evaluation a mean value has been calculated for every specimen.
Rotation about the x-axis represents varus/valgus movements, rotation about the y-axis dorsi-/plantarflexion, and rotation about the z-axis internal-/external rotation.
For statistical analysis ROM values from the Instron were compared to the Optotrak results (TIB-CAL) using the Wilcoxon signed-rank test for depended samples was used. The same procedure was used for statistical comparison within the Optotrak results.
Summary of the results from all quasi-static tests
p = 0.028 p = 0.046
p = 0.028
p = 0.028 p = 0.028
p = 0.028
p = 0.028 p = 0.028
p = 0.028
The mean range of motion for the flexion test was 5.3° (SD 1.45) seen from the material testing machine and 2.1° (SD 0.39) as seen from the Optotrak. This difference was statistically significant with p = 0.046. We found approximately a 3-times significant higher movement in the talocrural joint compared to the subtalar joint (p = 0.028, Fig. 7).
For varus/valgus and flexion data, the material testing machine data enabled determination of values in the neutral zone (Fig.6).
TTCA using intramedullary nails is an established salvage procedure. Unfortunately, it happens quite often that non-union complications of the upper ankle joint and/or the subtalar joint occur.
Therefore, biomechanical testing and the development of new fixation methods and designs are of significant interest also for TTCA. Starting with femoral nails for TTCA, new generations of intramedullary nails have been developed again and again.
Biomechanical testing of the new developed implants was conducted in several test setups ranging from static to cyclic loading, but all of them mainly using machine data for mechanical characterization [3, 4, 22–25]. Fragomen et al. investigated the Ilizarov external fixator and an intramedullary nail regarding micro motions. Therefore they used a passive marker optical measurement system above and under the osteotomy gap, but only a two dimensional evaluation was possible .
With our modifications of the test setup developed by Mückley et al., we were able to exclude shear stress on the construct and ensure a constant lever arm and consequently a standardized application of constant moments/forces to the construct [10, 11]. The additional use of the 3D motion measurement system allows the detection of movements between the involved bones with high precision. Utilizing a combination of material testing machine data and the Optotrak system, it is possible to characterize the ROM from both, the neutral zone (taken from the machine data) and the interbony movements (taken from the Optotrak system). This is possible by matching the load data of the machine with the motion data of the Optotrak.
For the HAN we found the talocrural joint prone to torsional stress as well as dorsiflexion and plantarflexion, whereas the subtalar joint is more prone to varus/valgus stress. Comparing the range of motion results from the machine and the optical system, we found no significant differences for torsional loading. In contrast, we found statistically significant differences for varus/valgus and dorsal/plantar flexion. These differences result from the test setup with using a lever arm for these loading scenarios. Therefore, the values from the machine are significant higher compared with the values from the optical measurement system looking at pure bony movements. The results of the neutral zone are only determined from the machine data and therefore, include the instability of implant and of the fixture.
Further important features of today’s intramedullary hindfoot nails are locking and compression mechanisms. Biomechanical studies have demonstrated that a higher stability can be attained when a combination with these fixation methods is employed [10, 11].
Using the presented setup biomechanical investigations of the hindfoot can be performed very sophisticated. The optical measurement system allows the differentiation of the involved joints (tibio-talar and talo-calcanear) combined with the machine data, that allow gathering information of the neutral zone and the whole construct. Overall, the setup allows detecting weak points of implants or effects of operative procedures. The test setup is not limited to hindfoot arthrodesis and intramedullary nails. With slight modifications implants/operations concerning the ankle joint or the subtalar joint and the effect to the other joint can be investigated.
Using the results strategies/developments can be performed to optimize implants or procedures to improve the management of hindfoot pathologies. Moreover, modifications to the implants themselves may allow a further reduction of the complication rates by increasing the stability of the TTCA construct.
This study also has limitations: using biomechanical testing without any soft tissue/muscle/tendon simulation is a basic method to compare mechanical characteristics of bone implant interfaces and limited cofounders. Additionally, the cartilage in talocrural and the subtalar joint has not been removed. This allowed us a very standardized approach resulting in a relative unstable situation. Another limitation is the independence of the optical and machine data, this does us not allow to calculate neutral zone values from the optical dataset.
The modified test setup for the hindfoot presented here promotes a more physiological and biomechanical loading. This offers – together with the 3D optical measuring system – a more standardized testing, above all for difficult cases, and also allows a detailed characterisation of the bone-implant constructs. Utilizing this, new implants and surgical techniques at the hindfoot can be compared with the current state of affairs; implant developments can be followed and, where necessary, their complications reduced.
HAN, hindfoot arthrodesis nail; NZ, neutral zone; ROM, range of motion; SD, standard deviation; TTCA, tibiotalocalcaneal arthrodesis
This study was supported by Small Bone Innovations Inc. (SBI, 1380 S. Pennsylvania Avenue, Morrisville, PA 19067). The Company delivered all of their implants, paid for the specimens used and for the implants of the competitor. Additionally, financial support for laboratory expenses was given.
Availability of data and materials
All the data supporting your findings is contained within the manuscript.
We confirm that all authors read and approved the final version of the manuscript. JE substantially designed the study and carried out data interpretation and drafting the manuscript. MS was responsible for the test setup's design and conception, as well as data analysis. DG undertook data acquisition, data interpretation and manuscript revision. ML also surveyed data acquisition and evaluation. MJR overtook data interpretation and critical manuscript revision. DW was responsible for data evaluation, statistics and manuscript drafting. SO supported instrumentation, design of the study and drafting of the manuscript.
The authors declare that they have no competing interests.
Consent for publication
Not applicable, this is a biomechanical study.
Ethics approval and consent to participate
The local institute of anatomy provided the specimens used in this study. The local Ethical Committee (Ethikkommission der Ärztekammer Westfalen-Lippe und der Medizinischen Fakultät der Westfälischen Wilhelms-Universität) stated before, that biomechanical studies using specimens from body donors do not need an approval.
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