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SECTION I: SYMPOSIUM: Papers Presented at the 2006 Meeting of the Knee Society

Soft Tissue Balance in Total Knee Arthroplasty: Does Stress Relaxation Occur Perioperatively?

Bellemans, Johan; D'Hooghe, Pieter; Vandenneucker, Hilde; Damme, Geert, Van; Victor, Jan

Section Editor(s): Laskin, Richard S MD, Guest Editor

Author Information
Clinical Orthopaedics and Related Research: November 2006 - Volume 452 - Issue - p 49-52
doi: 10.1097/01.blo.0000238790.29102.95

Abstract

Restoring adequate soft tissue balance is one of the important factors in obtaining a successful total knee arthroplasty (TKA). Leaving the knee too loose after arthroplasty may theoretically lead to tibiofemoral instability, whereas excessive tightness may cause stiffness.5,10-12,27 Recent work has demonstrated tibiofemoral instability was associated with failure in 21% to 29% of knees undergoing revision, whereas persistent stiffness was present in 17% of patients needing early revison.17,20 Retrospective clinical and biomechanical studies have demonstrated inadequate soft tissue balance is associated with an increased incidence of radiolucent lines at the tibial-bone interface, negatively influences tibiofemoral and patellofemoral stresses, affects the pattern and severity of polyethylene wear, and causes loss of joint proprioception.2,3,19,21,26,28 However, these retrospective studies only suggest an association and no prospective study confirms whether appropriate soft tissue balancing will affect the long term outcome of knee arthroplasty.

Most surgeons believe some mediolateral laxity should be achieved postoperatively, with the ideal knee being somewhat looser in flexion than in extension, and somewhat looser laterally than medially.2,9,23,24,27 Unfortunately, there is no plausible data confirming these beliefs, and until recently there has been no intraoperative method to measure the soft tissue balance. The latest generation computer navigation systems have now however reached sufficient precision to allow such measurements, generating interest among knee surgeons and engineers in using this technology for ligament balancing during TKA.14,18,24 Modern navigation systems have an accuracy of 0.19° for rotations and 0.35 mm for translations, and can be used to measure the mediolateral joint line opening during varus and valgus stress testing, or any other laxity test.4,24,25

When the surgeon knows the reference values, the operated knee could in theory be balanced with the same laxity or tightness as a natural knee. Our group has previously reported normative laxity data obtained in normally aged, nonarthritic knees using computer navigation technology.24 These data may serve as a guideline during surgery when assessing mediolateral joint line opening during varus/valgus stress testing, or when assessing anteroposterior and rotational laxity, passive extension and maximal passive hyperextension.24

Even if the optimal soft tissue tension were known, it would not always be attainable, and knees are often too tight or too loose depending on insert thickness.2,9,27 Many surgeons accept a slightly tighter than looser knee, as some soft tissue stress relaxation is expected which may cause some further loosening.1,13,16,22 Given the viscoelastic nature of soft tissues, constraining tissues around the knee that appear too tight immediately after component implantation likely relax over time so the knee feels more normal after surgery. The degree of such soft tissue relaxation has not, however, been accurately documented or quantified.

We hypothesized some degree of soft tissue relaxation occurs in the early phase after TKA component insertion.

MATERIALS AND METHODS

Twenty-five standard TKA procedures were analyzed in this study. A cemented posterior stabilized Genesis II prosthesis® (Smith and Nephew, Memphis, TN) was employed in all cases, using the instrumentation provided by the implant manufacturer. Surgery was performed using computer navigation (iON®, Medtronic SNT, Louisville, CO), with an active frame attached to the femur 10 cm above the knee joint, and a second passive frame attached to the tibia in the mid-diaphyseal region.

Once the desired implant position, component alignment and soft tissue balance was achieved, all components were cemented in situ and the polyethylene insert with the appropriate thickness was introduced. The tourniquet was released exactly 15 minutes after cement mixing and hemostasis was performed.

Mediolateral laxity measurements were performed using manual varus/valgus stress testing in extension and in 90° flexion. Maximal joint line opening on the medial and lateral side was recorded, using the iOn System® (Medtronic SNT, Louisville, CO) and FluoroKnee Software® (Smith & Nephew, Memphis, TN). Range of motion was also measured using the computer navigation system. Maximal “drop and dangle” flexion was recorded while manually supporting the femur only. Maximal passive extension was measured while extending the limb by the foot. All measurements were obtained upon one tenth of a degree using the FluoroKnee Software® (Smith & Nephew, Memphis, TN) adapted for the iOn System® (Medtronic SNT, Louisville, CO). All measurements were performed by the same surgeon with a high volume experience in TKA.

All measurements were repeated 30 minutes later under identical conditions. In the meantime, the knee was left untouched in extension on the operating table. Measurements were performed while the arthrotomy was approximated using two or three towel clips.

All data were stored in an Excel XP® database (Microsoft Corp, Redmond, WA) and analyzed using Analyse-it plug-in software (Version 1.71; Analyse-it Software, Leeds, UK). We confirmed normality of the data with a Shapiro-Wilk test and two-sided paired Student t-tests compare laxity immediately after implantation and 30 minutes later.

RESULTS

We observed stress relaxation during the first 30 minutes after implantation in all cases. The intraoperative measurements indicated stress relaxation caused increased (p < 0.001) laxity for all measurements (except maximal flexion) at 30 minutes compared with immediately after implantation. The medial joint line opening on valgus stress increased (p < 0.001) by an average of 0.95 mm (SD = 0.097) in extension and 0.99 mm (SD = 0.094) in flexion. Lateral joint line opening on varus stress increased (p < 0.001) by an average of 1.05 mm (SD = 0.099) in extension and 1.01 mm (SD = 0.11) in flexion. Maximal passive extension increased (p < 0.001) by an average of 3.04° (SD = 0.28) (Table 1). Maximal “drop and dangle” flexion did not change.

TABLE 1
TABLE 1:
Effects of Stress Relaxation (Mean and Standard Deviation)

DISCUSSION

Assessing the immediate technical operative results after TKA has traditionally been based on radiographic parameters related to implant position and alignment.7,8,15 The implant-soft tissue interaction has been neglected because there was not an adequate method of measurement and registration.2,9,12,18,24,27 Most surgeons agree soft tissue balance is one of the most difficult steps during TKA.2,6,9,12,18,24,27 It takes a long time to learn whether the knee is adequately balanced and not too tight or too loose. Using computer navigation technology, one can make an accurate assessment of mediolateral joint line opening and other variables relating to the soft tissue status after TKA.14,24

Our study has some limitations inherent to the measurement techniques used. Manual stress testing was performed to assess mediolateral laxity and, despite having been performed by an experienced surgeon, some degree of variability in the forces exerted during testing can not be excluded. Nevertheless, in every case a certain amount of increased joint line opening was noted, and also maximal passive extension-which was assessed without active stress being imposed by the investigator-increased in every case. The measurements in our study were performed until 45 minutes after cementation (30 minutes after cement hardening), and were not continued thereafter for reasons of time constraint and to avoid additional exposure of the patient. Additional laxity from tissue adaptation or remodeling may, however, occur in the later stages. Some evidence for this is derived from reports on patients with residual flexion contractures after TKA, which have a tendency to improve up to 1 year or more after surgery.1,13,16,22 Further studies will therefore be needed to clarify how soft tissue status evolves at a later stage after TKA.

The status achieved immediately after surgery may, however, alter over time, and it is unclear whether some type of soft tissue relaxation occurs in the early phase after surgery. This hypothesis was investigated in our work, and we demonstrated soft tissue stress relaxation occurred during the first 30 minutes after TKA implantation.

As a consequence, mediolateral opening on varus/valgus stress increased by an average of approximately 1 mm on both the medial and lateral side, and maximal passive extension increased by an average of 3°.

Data obtained using the same technology in normal, nonarthritic knees allows the comparison of the situation obtained during surgery with reference values as documented in the normal knee (Table 2).24 These data suggest medial or lateral laxity in extension in the range of 2 to 3 mm, while our data is in the range of 1 mm. The normative data was obtained on cadaveric specimens with a moment of 9.8 Nm, and either the quality and state of the specimens and amount of loading may explain the differences in that data and ours.

TABLE 2
TABLE 2:
Reference Values in Normal Knees* (mean and 95% confidence interval)

Our data may have implications towards clinical practice. In situations where the surgeon is confronted with a knee that appears too tight or too loose depending on the insert thickness he or she chooses, the option to consider the thicker insert and to accept an initially slightly tighter than looser knee is defendable, since some soft tissue stress relaxation can be expected, which may cause some further loosening-up of the joint. For the same reason it may be defendable to accept a small extension deficit of a few degrees after TKA implantation.

Our data demonstrate soft tissue stress relaxation does occur in the early phase after TKA, leading to approximately 1 millimeter of extra joint line opening on varus/valgus stress testing, and an increase in maximal passive extension of approximately 3° during the first 30 minutes after surgery.

References

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