Research & Validation
Our research so far on muscle dynamics in arthroplasty
Formus Navigator is a software application in development that harnesses advanced biomechanical analysis of soft-tissues to support pre-, intra-, and post-operative decision-making in arthroplasty. Patient-specific musculoskeletal models are automatically generated from imaging data1 and interrogated to elucidate how surgical targets influence the function of the hip throughout motion.
*patent pending
These data are distilled into a soft-tissue safe zone* that can be interactively reviewed, allowing surgeons to optimise their surgical targets preoperatively, and adapt intraoperatively to maximise patient outcomes.
We have known for decades that the force- and moment- generating capacities of the hip muscles are sensitive to the location of the hip centres2. Cup implantation can alter the 3D position of the hip joint centre (HJC), while stem design parameters (e.g. neck length, neck-stem angle) and implantation parameters (e.g. anteversion, depth) can alter the 3D position of the femoral head centre (FHC).
Deviations to both the HJC and FHC have permanent and patient-specific implications for the biomechanics of the joint throughout motion3-11, highlighting the need to optimise surgical targets for each patient.
%402x.svg){kind=icon}
%402x.svg){kind=icon}
Isolated medialization of the HJC trades adduction capacity for abduction capacity of muscles crossing the hip throughout motion3, 4. However, the effects are patient-specific and certain patients benefit more than others4-6.
For example, superiorizing and medializing the HJC can counter the benefits of medialisation for the tensor fascia latae during the lean-forward phase of sit-to-stand3. The combined effect of these 3D deviations on each patient's biomechanics can only be determined through patient-specific simulation.
Increased femoral offset increases abduction strength7, 8. Increasing stem head-offset increases both offset and leg length simultaneously. This enhances abductor moment arms without impairing adductor moment arms, however, this also increases peak muscle lengths across all muscles9, 10, and can risk pain from muscle overstretch.
Stem anteversion has variable implications for gluteus muscle function: increasing anteversion can improve external rotation function. However, it can also lengthen and slacken the gluteus muscles at different phases of hip motion11, risking both pain and weakness during motion.
In reality, 3D deviations are not isolated, and deviations to the HJC, offset, leg length, and anteversion must be considered alongside patient-specific anatomy to minimize risks and optimize post-operative function.
Formus Navigator's patient-specific musculoskeletal models can quantify the functional changes in the hip in order to provide surgeons with new tools to assess and optimize surgical targets for each patient to avoid overstretching and weakening of the soft-tissues.
Formus Navigator is a cloud-based software application that gives novel insights about patient-specific soft-tissue biomechanics. State-of-the-art AI is used to automatically obtain patient-specific anatomy from either CTs or two X-rays, and generate a fully articulated patient-specific musculoskeletal model of the lower limbs.
This digital twin is used to simulate every day motions and quantify the effects that surgical decisions, such as changes to joint centers, have on estimates of muscle function.
These data are distilled into an intuitive soft-tissue safe zone for each implant which provides objective guidance on surgical targets during planning and execution. Intraoperatively, delivery information are used to update the model, rerun the simulations, and recalculate the safe-zone. Post-operatively, the soft-tissue analysis can be used to guide patient recovery.
1. Rooks, N., Bakke, D., Schneider, M., Besier, T. (2023). Automated generation of patient-specific musculoskeletal models to predict muscle function following total hip arthroplasty. In International Society of Technology in Arthroplasty 2023 Annual Congress. New York, New York, USA.
2. Delp, S. L., & Maloney, W. (1993). Effects of hip center location on the moment-generating capacity of the muscles. Journal of biomechanics, 26(4-5), 485-499.
3. Kamath, A., Liu, D., Bakke, D., Schneider, M., Besier, T. (2023). Medialization of the Acetabular Cup impacts hip abductor function. Poster session presented at International Society of Technology in Arthroplasty 2023 Annual Congress. New York, New York, USA.
4. Kamath, A., Mason, J. B., Bakke, D., Liu, D., Germano, J., Besier, T., Schneider, M. (2024). Effects of acetabular cup medialization on muscle function vary between THA patients. In International Society of Technology in Arthroplasty 2024 Annual Congress. Nashville, Tennessee, USA.
5. Terrier, A., Florencio, F. L., & Rüdiger, H. A. (2014). Benefit of cup medialization in total hip arthroplasty is associated with femoral anatomy. Clinical Orthopaedics and Related Research®, 472(10), 3159-3165.
6. Terrier, A., Parvex, V., & Rüdiger, H. A. (2016). Impact of individual anatomy on the benefit of cup medialisation in total hip arthroplasty. Hip International, 26(6), 537-542.
7. McGrory, B. J., Morrey, B. F., Cahalan, T. D., An, K. N., & Cabanela, M. E. (1995). Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. The Journal of Bone & Joint Surgery British Volume, 77(6), 865-869.
8. Liu, D., Schneider, M., Bakke, D., Besier, T. (2023). Some hip abductor muscles are more susceptible to femoral offset changes following total hip arthroplasty. In AOA 2023 Annual Scientific Meeting. Melbourne, Australia.
9. Germano, J., Liu, D., Schneider, M., Bakke, D., Besier, T. (2023). Adductor Muscle Secrets: The Impact of Femoral Offset on Muscle Tension during Sit-to-Stand Motion. Poster session presented at American Association of Hip and Knee Surgeons 2023 Annual Meeting. Dallas, Texas, USA.
10. Mason, J. B., Bakke, D., Liu, D., Germano, J., Kamath, A., Besier, T., Schneider, M. (2024). Femoral head offset effects on abductor muscle function vary between THA patients. In International Society of Technology in Arthroplasty 2024 Annual Congress. Nashville, Tennessee, USA.
11. Petterwood, J., Bakke, D., Schneider, M., Liu, D., Besier. (2023). Femoral stem version alters the internal-rotation function of gluteal muscles. In AOA 2023 Annual Scientific Meeting. Melbourne, Australia.
Join the community that brings Formus Navigator to life and stay up to date with the our progress
Register Now
