The ligaments of the knee joint are responsible for assisting in the transduction of force from the femur to the tibia and guiding the long bones of the leg through the cyclic motions of everyday activities.
These soft tissues operate within the constraints of finite physical properties and behaviors that describe their ability to withstand cyclic loading. One of these physical behaviors is called “viscoelasticity” and describes the deformation of soft-tissue due to applied load over time.6 Ligaments of the human body follow a non-linear viscoelastic curve, indicating that they will initially stretch to accommodate an applied load, but return to their original shape when the load ceases, much like a rubber band. However, each ligament has a biological maximum threshold for applied load – loading beyond this threshold will contribute to a permanent deformation of the soft-tissue and potential laxity or failure of the ligament itself.7
As the human body develops from childhood to adulthood, the collagen fibrils within ligaments increase in diameter and thus increase the relative stiffness and strength of the ligament itself. The larger relative strength of the ligament thereby increases the maximum viscoelastic threshold.6 But, as the human body continues to age, the collagenous fibrils of the soft tissues begin to deteriorate and a proportional loss of strength is observed in the ligaments of knee joint as well. In an older population of patients, anatomic changes associated with late-stage osteoarthritis can therefore adversely stretch or contract these ligamentous structures. For instance, as the cartilage of the knee collapses, an asymmetric and unfavorable varus or valgus angle is induced between the tibia and femur. This unnatural malalignment inappropriately loads the surrounding ligaments, the ligaments begin to exceed their viscoelastic maximum thresholds, and the strength of the ligaments can be compromised. This leads to an unstable and painful knee joint.
When performing a total knee arthroplasty, it is the job of the surgeon to not only correct osteoarthritic bony damage, but to also re-balance the stretched and strained ligaments. This correction of the ligamentous tissues of the joint is critical. Because the soft tissues of the knee guide the long bones of the leg through the swing and stance phase, residual imbalance can adversely affect the gait of a patient, even after bony deformity has been corrected.
Imbalanced soft tissues after can present as chief complaints of pain, instability, or restricted range of motion.1-3 If uncorrected, these symptoms of imbalance can limit the patient’s functional abilities, unfavorably affect their quality of life, and may even result in the need for additional surgery.1,5
Because the correction of these complications often requires surgeon or therapist intervention, imbalanced primary total knee arthroplasties are a substantial contributing factor to the financial burden associated with revision procedures. Currently, over half of early and late-stage revision TKA surgery can be attributed to soft tissue imbalance – by 2030, it is estimated that these revision procedures will account for 13 billion dollars of annual revision burden.3,4,8
Historically, surgeons have implemented several techniques to judge soft tissue balance in total knee arthroplasty. Unfortunately, these techniques often rely on the subjective opinion of the surgeon, cannot quantify the loading of the compound action of medial and lateral structures, and are only able to be employed in static positions of the knee itself. Due to these limitations, balanced results are difficult to replicate and a large degree of variability may be seen in the state of balance between patients. For these reasons, the rate of soft tissue imbalance related complications is observed within a wide range, from 26% to over 50% of complications reported in current literature.9-11
As hospitals begin to adopt a bundled payment schema for reimbursement, avoidance of soft tissue imbalance related complications will be paramount to the success of the surgeon, the satisfaction of the patient, and compliance of the hospital with new CMS guidelines.12
1 Bozic K, Kurtz S, Lau E, et al. The epidemiology of revision total knee arthroplasty in the united states. Clin Orthop Relat Res. 2010. 468: 45-51.
2 Thiele K, Perka C, Matziolis G, Mayr HO, Sostheim M, Hube R. Current failure
mechanisms ager knee arthroplasty have changed: polyethylene wear is less common in revision surgery. J Bone Join Surg. 2015; 97(9): 715-720.
3 Lombardi AV, Berend KR, Adams JB. Why knee replacements fail in 2013: patient, surgeon, or implant? Bone Joint J. 2014;96-B(11 Supple A): 101-104.
4 Bhandari M, Smith J, Miller L, et al. Clinical and economic burden of revision knee arthroplasty. Clin Med Insights Arthritis Musculoskelet Disord. 2012. 5: 89-94.
5 Hofmann S, Seitlinger G, Djahani O, Pietsch M. The painful knee after tka: a diagnostic algorithm for failure analysis. Knee Sports Surg Traumatol Arthrosc. 2011; 19: 1442-1452.
6 Hollister, S. “Ligament/Tendon Structure-Function”. BME332 Introduction to Biosolid Mechanics. Univeristy of Michigan – Department of Biomedical Engineering; Retrieved on September 18, 2016, from: http://www.umich.edu/~bme332/ch10ligten/bme332ligamenttendon.htm
7 Provenzano P, Lakes R, Keenan T, Vanderby R. Nonlinear ligament viscoelasticity. Annals Biomed Eng. 2001; 29: 908-914.
8 Bhandari M, Smith J, Miller LE, Block JE. Clinical and economic burden of revision knee arthroplasty. Clin Med Ins. 2012; 5: 89-94.
9 Lombardi AV, Berend KR, Adams JB. Why knee replacements fail in 2013: patient,surgeon, or implant? Bone Joint J. 2014;96-B(11 Supple A): 101-104.
10 Schroer WC, Berend KR, Lombardi AV, et al. Why are total knees failing today? Etiology of total knee revision in 2010 and 2011. J Arthroplasty 2013;28(8 Suppl):116–119.
11 Leta TH, Lygre SH, Skedderstuen A, Haalan G, Furnes O. Failure of aseptic revision in total knee arthroplasties. Acta Orthop. 2015; 86(1): 48.
12 Department of Health and Human Services. “CMS finalizes bundled payment initiative for hip and knee replacements”. HHS Press Office. November 16, 2015; Accessed at: http://www.hhs.gov/about/news/2015/11/16/cms-finalizes-bundled-payment-initiative-hip-and-knee-replacements.html.