The purpose of the study by Roth et al. was to evaluate the tissue laxities of the native knee in cadaveric specimens and to determine whether these were equal at 0°, 45°, and 90° of flexion. By doing this evaluation, the authors sought to determine whether the laxities observed at these angles in the cadaveric knee supported the use of gap-balancing in total knee arthroplasty. Historically, gap-balancing was pioneered with the goal of equalizing medial and lateral soft-tissue tension and creating a rectangular space in flexion and extension. With the aim of correcting varus or valgus knee deformities, gap-balancing has since been accepted and used as one of the conventional methods for performing total knee arthroplasty. However, intraoperatively, balancing is often only performed at 0° and 90° of flexion and does not account for soft-tissue balance throughout the arc of motion. Therefore, these ligaments may be slack at an intermediate point in the arc of flexion, leading to flexion instability, an important cause of postoperative patient dissatisfaction. Studies such as this one are critical now as they lead us to examine why this occurs, particularly if gap-balancing has been widely accepted as a standard method. If such instability may be related to unrecognized ligament laxity during surgery, then this method may be failing to address something. To ensure the continued success of knee arthroplasty, it is necessary to identify the causes of patient dissatisfaction and complications associated with soft-tissue restraints, particularly as reimbursements from the Centers for Medicare & Medicaid Services are dependent on postoperative outcomes and readmission rates1. Therefore, studies such as this one are pertinent in providing us with a fresh perspective on accepted surgical techniques.
In the current study, ten fresh-frozen native cadaveric knees with intact menisci, cartilage, and ligaments were used. The authors excluded specimens with evidence of degenerative arthritis or chondrocalcinosis. With the use of a six-degrees-of-freedom load application system, seven different laxities were assessed at 0°, 45°, and 90° of flexion by applying loads of ±5 Nm in varus-valgus rotation, ±3 Nm in internal-external rotation, 100 N in distraction, and ±45 N in anterior-posterior translation. At 45° compared with 0° of flexion, the average laxities ranged from 0.9° greater in valgus (p = 0.0002) to 10.2° greater in internal rotation (p < 0.0001). At 90° compared with 0° of flexion, the average laxities ranged from 1° greater in valgus (p = 0.0001) to 10.1° greater in external rotation (p < 0.0001). They concluded that the laxities of the native knee are not equal and show great variability throughout the arc of motion. The authors alluded to the possibility that current techniques to create equal laxities may not allow for the reinstatement of native kinematics following total knee arthroplasty. They suggested that total knee arthroplasties using kinematic alignment may better restore the laxities of the native knee.
Current techniques, i.e., gap-balancing, only look at tissues at 0° and 90° of flexion, while newer techniques, i.e., kinematic alignment, consider the three-dimensional component alignment with respect to the knee and incorporate three axes: two transverse axes in the femur, about which the tibia and patella flex and extend, and a longitudinal axis in the tibia, about which the tibia internally and externally rotates on the femur. The main difference with kinematic alignment is that components are aligned with respect to the weight-bearing line, and the obliquity and level of the joint line are restored anatomically2. Dossett et al. conducted a randomized controlled trial comparing functional outcomes of mechanically aligned and kinematically aligned total knee arthroplasties in eighty-eight patients3. They noted that at the two-year follow-up, functional outcomes were better in the kinematically aligned group, and they also reported higher mean flexion compared with the mechanically aligned cohort (121° versus 113°; p = 0.002).
Despite good intraoperative gap-balancing, patients often complain that their knees do not feel normal. This leads us to question whether this feeling may be a consequence of malalignment that does not correspond to that of the normal knee. With dissatisfaction rates as high as 28% following surgery4, this study is important and its findings are supported by several investigations that have demonstrated less than desirable outcomes with standard gap-balancing techniques. Gu et al. evaluated the frequency of ligament imbalances and malalignment following gap-balancing on fifty three-dimensional bone models5. Those authors noted that 34% of the models required release of a tight medial collateral ligament and 30% required release of a tight lateral collateral ligament. In addition, 56% of models demonstrated >2 mm of instability between 0° and 90° of flexion in the medial compartment. With standard gap-balancing techniques, surgeons should be aware of the risk of ligament imbalances.
On the basis of the findings of this study, achieving equal laxities may not be representative of the native knee, which may explain the persistence of postoperative dissatisfaction.
As with all studies, this investigation has limitations. First, it is a cadaveric study that incorporates only ten specimens. Although the study power was appropriate, the sample size is small and assessment of more specimens may provide a better picture of native knee laxities. Second, with the use of cadavers, artificial load was applied, and this may not represent physiologic load, which is likely to vary among patients. Third, we cannot currently predict whether these findings are associated with clinical outcomes. Despite these limitations, the authors are to be applauded for their work; this study calls for critical evaluation of current methods of soft-tissue balancing in total knee arthroplasties.
1. Hackbarth G, Reischauer R, Mutti A. Collective accountability for medical care—toward bundled Medicare payments. N Engl J Med. 2008 Jul 3;359(1):3-5.
2. Howell SM. Kinematic vs. mechanical alignment: What is the difference? Orthopedics Today. October 2010.
3. Dossett HG, Estrada NA, Swartz GJ, LeFevre GW, Kwasman BG. A randomised controlled trial of kinematically and mechanically aligned total knee replacements: two-year clinical results. Bone Joint J. 2014 Jul;96-B(7):907-13.
4. Khatib Y, Madan A, Naylor JM, Harris IA. Do psychological factors predict poor outcome in patients undergoing TKA? A systematic review. Clin Orthop Relat Res. 2015 Aug;473(8):2630-8. Epub 2015 Mar 20.
5. Gu Y, Roth JD, Howell SM, Hull ML. How frequently do four methods for mechanically aligning a total knee arthroplasty cause collateral ligament imbalance and change alignment from normal in white patients? AAOS Exhibit Selection. J Bone Joint Surg Am. 2014 Jun 18;96(12):e101. Epub 2014 Jun 18.