J Orthop Sci (2007) 12:149–153DOI 10.1007/s00776-006-1108-8

Original article

Soft tissue balance measurement in anterior cruciate ligament-resected knee joint: cadaveric study as a model for cruciate-retaining total knee arthroplasty

Koichi Tanaka1, Hirotsugu Muratsu1, Kiyonori Mizuno1, Ryosuke Kuroda1, Shinichi Yoshiya2, and Masahiro Kurosaka1

1 Department of Orthopedic Surgery, Kobe University Graduate School of Medicine, 7-5-2 Kusunoki-cho Chuo-ku, Kobe 650-0017, Japan2 Department of Orthopedic Surgery, Hyogo College of Medicine, Hyogo, Japan

cases (27%) were caused by instability.1 They concluded that the instability might be due to inadequate correc-tion of soft tissue imbalances in both sagittal and coro-nal planes. As a result, soft tissue balancing has been recognized as an essential component of the surgical procedure for improving outcomes after TKA.

However, soft tissue balancing remains a diffi cult issue during surgery, as much depends on the surgeon’s “feel.”2–4 Conventionally, after osteotomy an identical extension and fl exion gap with no inclination between the medial and the lateral gap between bony surfaces has been generally accepted as the most appropriate for soft tissue balance during the TKA procedure.5 How-ever, some experienced surgeons have noted diffi culties in matching these balances and have begun to suggest that the fl exion gap may not be rectangular as previ-ously thought.6,7

In response to such an ambiguous problem, a growing number of surgeons have turned to computer-assisted navigation technology.8–10 Among variable navigation systems, surgeons seek the system with an option to help them do a better job at soft tissue balancing during TKA. Recently, some systems have been developed with special software modules to estimate soft tissue balance through a range of fl exion with the trial implant in place during surgery. Despite such a technological advancement, we still do not have a defi nitive answer to the ultimate status of soft tissue balancing to be aimed at during the procedure. In general, the goal of any arthroplasty is to correct the pathology of the joint and reestablish the physiological condition.

The purpose of the present study was to determine the soft tissue balance in an anterior cruciate ligament (ACL)-resected normal knee joint throughout the range of knee fl exion that can provide reference data for cru-ciate-retaining TKA. To this end, we defi ned the “joint gap” as the distance between the articular surfaces of the knee joint under a quantitative joint distraction force, independent from the thickness of the resected

AbstractBackground. The soft tissue balancing procedure remains a diffi cult issue during total knee arthroplasty, as much depends on the surgeon’s “feel.” Although computer-assisted naviga-tion technology has been attempting to evaluate the joint stability, we have no defi nitive answer to an ideal soft tissue balance of the knee joint. The purpose of the present study was to determine the soft tissue balance in an anterior cruciate ligament (ACL)-resected normal knee joint throughout the range of knee fl exion, which may provide reference data for cruciate-retaining total knee arthroplasty (TKA).Methods. We investigated joint stability in 10 ACL-resected normal cadaver knees throughout the range of fl exion under consistent joint distraction force using a specially developed tensioning device for TKA. We measured both medial and lateral joint gaps as the separation distance between the ar-ticular surfaces with 40 lb (18.7 kg) of joint distraction force.Results. Both medial and lateral joint gaps at 0° of fl exion were signifi cantly smaller than those at other fl exion angles. The medial joint gap was almost consistent during knee fl ex-ion; however, the lateral joint gap increased with knee fl exion and showed a signifi cantly larger value at 60°–120° of fl exion than the medial joint gap.Conclusions. These characteristics of joint stability in the ACL-resected normal knee need to be taken into consider-ation in soft tissue balancing during cruciate-retaining TKA.

Introduction

Total knee arthroplasty (TKA) is a well-established procedure that generally results in pain relief, improved physical function, and a high level of patient satisfac-tion. However, instability after primary TKA is one of the most common causes of early TKA failure. Fehring et al. studied 279 revision surgeries within 5 years of their index arthroplasty and reported that 74 revision

Offprint requests to: K. TanakaReceived: July 18, 2006 / Accepted: December 12, 2006

150 K. Tanaka et al.: Soft tissue balance for CR TKA

bone of the distal femur and the proximal tibia. The joint gap at both the medial and lateral sides of the knee joint in normal cadaver specimens throughout the range of knee fl exion was evaluated.

Furthermore, although several methods and devices for assessing soft tissue balance (e.g., manual distrac-tion6 and the use of spacers,5 tensors,5,11–13 and electric instruments14,15) have been described, these devices ne-cessitated the measurement under nonphysiological joint conditions without a prosthesis and only at exten-sion and 90° of fl exion. A specially developed tension-ing device was used in the study to apply quantitative joint distraction force under physiological joint status and simultaneously measure the medial and lateral joint gap throughout the range of knee fl exion.

Materials and methods

This study was performed at the Clinical Training & Education Centre at the University of Western Austra-lia (Perth, Australia). Following Institutional Review Board approval, 10 fresh cadaveric knees were obtained. All knees were free of apparent osteoarthritic change or deformity. The knees were from six men and four women with ages ranging from 72 to 92 years.

Each knee was harvested and stored for freezing at −10°C. at 24 h before the experiment, the sample was removed from the freezer to ensure thawing. During the testing, the room temperature was kept constant at around 25°C, and the examined knee was kept moist with a saline-moistened sponge.

The knees were exposed with a medial parapatellar arthrotomy, and the ACL was resected. A proximal tibial osteotomy was performed perpendicular to the long axis in the coronal plane and at a 7° posterior slope in the sagittal plane using an intramedullary alignment rod as performed in the tibia fi rst technique in a cruci-ate-retaining TKA. At the osteotomy, a bony block at the tibial insertion site of the posterior cruciate ligament was preserved, as in the actual TKA procedure, and soft tissue release was limited within the extent of the bone resection. After the tibial bone resection, the newly de-veloped tensioning device was introduced to the space between the osteotomized tibia and the intact articular surface of the femoral condyles.

The newly developed tensioning device consists of three parts: upper seesaw plate, lower platform plate, and extraarticular main body. Two plates are connected to the extraarticular main body by the offset connection arm through a medial parapatellar arthrotomy, which allows patellofemoral (PF) joint reduction during the measurement. The tensioning device can be fi rmly fi xed to the center of the osteotomized tibia by spikes and additional pins. The lower platform plate was positioned

and aligned based on the shape of the tibial osteoto-mized surface and the location of anatomical landmarks, such as the insertion of the posterior cruciate ligament and tibial tubercle. The seesaw plate can move in a proximal-distal direction by means of a rack and pinion mechanism in the main body. Furthermore, the central-izer insert was affi xed to the upper surface of the seesaw plate to control mediolateral joint alignment. Thus, dur-ing the testing, sagittal as well as coronal alignment was maintained. On the other hand, the fl exion angle was kept constant at the predetermined value while inter-nal-external rotation was relatively unconstrained. The designated joint distraction force can be exerted be-tween the seesaw plate and the platform plate from 30 lb (13.6 kg) to 80 lb (36.3 kg) using a specially made torque driver. In the preliminary experiment, the error for the joint distraction force was demonstrated to be within ± 3%. For the soft tissue balance measurement with the PF joint reduced, the medial parapatellar arthrotomy was repaired by temporary stitches both proximally and distally to the offset connection arm of the tensor.

This tensioning device provides two scales: the center gap (in millimeters) between the upper surface of the seesaw plate with the centralizer insert and the under-surface of the platform plate, and the inclination angle (in degrees), which is a positive value in varus imbal-ance, between two plates. Therefore, this device pro-vides quantitative soft tissue balance assessment by both center gap and ligament imbalance under a desig-nated joint distraction force (Fig. 1).

In the present study, joint distraction force was ap-plied between the osteotomized tibia and the intact femoral condyles and was set at 40 lb (18.7 kg) because our preliminary clinical study had shown that the joint gap with 40 lb of distraction force at full extension most closely corresponded to the thickness of the insert actu-ally selected at the procedure. The measurements were carried out at six fl exion angles: 0°, 10°, 30°, 60°, 90°, and 120° as measured by a goniometer for 10 knees. Both the gap (millimeters) at the center of the joint and the inclination angle indicating ligament imbalance (de-grees in varus) were measured between the platform plate (tibial osteotomized surface) and the seesaw plate (femoral articular surface). The joint distraction force was loaded several times until the gap remained at a certain value to reduce the effect of creep elongation of the surrounding soft tissues. Measurements were re-peated three times, and the averages of these values were adopted.

The gaps at the medial and lateral compartments of the joint based on the tibial osteotomized surface were calculated and compared. Then the joint gaps between the tibial and femoral joint surfaces were calculated as follows. First, both medial and lateral thicknesses of the resected proximal tibia were calculated by the summa-

K. Tanaka et al.: Soft tissue balance for CR TKA 151

t-test at each fl exion angle. Differences of P < 0.05 were considered statistically signifi cant.

Results

The results of the measured and calculated values are shown in Tables 1 and 2 (mean ± SD). The mean lateral gaps based on the tibial osteotomized surface were sig-nifi cantly larger than the medial gaps throughout the range of motion (P < 0.01). The mean thickness of the resected bone of the proximal tibia was 4.4 ± 1.0 mm at the medial compartment and 10.0 ± 1.0 mm at the lateral compartment.

The joint gap (separation distance between the origi-nal joint surfaces) varied depending on the fl exion an-gles. Both medial and lateral joint gaps at 0° of fl exion were signifi cantly smaller than those at other fl exion angles (P < 0.01). There were no signifi cant differences among the medial joint gaps from 10° to 120° of knee fl exion. On the other hand, the lateral joint gap at 90° of fl exion was signifi cantly larger than that at 10° of fl exion. There were no signifi cant differences between the medial and lateral joint gap from 0° to 30° of fl exion.

Fig. 2. a Specially developed tensioning device enabling gap measurement with a certain amount of joint distraction force was introduced into the space between the surface of the os-teotomized tibia and the intact articular surface of the femur. b The gap based on the tibial osteotomized surface was mea-sured at the medial side (M1–M3) and the lateral side (L1–L3) by applying a distraction force with the tensioning device. The joint gap, defi ned as the separation distance between the origi-nal joint surfaces (L1–L2 and M1–M2), was also calculated from the thickness of the resected tibial bone

Fig. 1. Newly developed tensioning device

tion of the measured thickness of the resected bone us-ing a microcaliper at the center of each compartment and the thickness of a bone saw (1.25 mm). The joint gap was defi ned as the separation distance between the original joint surfaces with 40 lb (18.7 kg) of joint dis-traction force and was calculated by subtracting the thickness of the tibial bone resection from the measured gap values at both medial and lateral compartments (Fig. 2).

The results were analyzed statistically using a statisti-cal software package (Statview 4.0; Abacus Concepts, Berkeley, CA. USA). Comparisons of gaps at the medial and lateral compartments were made, respec-tively, using the repeated measures analysis of variance (ANOVA) among the knee fl exion angles. Post hoc analyses were performed by Fisher’s PLSD test. The differences in gap between the medial and lateral com-partments were analyzed using the paired Student’s

Table 1. Measured values throughout the range of knee fl exion

0° 10° 30° 60° 90° 120°

Center gap (mm) 10.6 ± 0.9 14.4 ± 0.5 15.2 ± 1.3 15.8 ± 1.6 16.2 ± 1.8 15.4 ± 1.5Inclination angle (°; varus) 4.2 ± 1.3 6.6 ± 2.1 7.7 ± 2.9 9.9 ± 2.4 10.8 ± 2.2 9.0 ± 1.7

152 K. Tanaka et al.: Soft tissue balance for CR TKA

However, the lateral joint gaps were signifi cantly larger than the medial joint gaps at 60° to 120° of fl exion (P < 0.05) (Fig. 3).

Discussion

We evaluated the joint separation distance between joint surfaces at the medial and lateral compartments with a consistent joint distraction force of 40 lb (18.7 kg) to estimate the inherent characteristics of joint stability in the ACL-resected normal knee joint. We found sev-eral characteristic features in joint stability that could provide reference data when considering the soft tissue balance during cruciate-retaining TKA.

First, we found that joint stability varied depending on the knee fl exion angle. Both medial and lateral joint gaps at 0° of fl exion were signifi cantly smaller than those at other fl exion angles, which meant that the knee became most stable at full extension.

We hypothesize the mechanism for explaining the least joint gap at 0° of fl exion as follows; the joint gap is signifi cantly reduced at 0° of fl exion owing to the ten-sion of the supporting structures, including the posterior capsule of the joint stretched by the posterior condyles of the femur, which might prevent hyperextension. As a result, marked expansion of the joint gap occurs even

with a slight fl exion movement. Mihalko et al. stated that the release of more posterior structures had a greater effect on the extension gap than on the fl exion gap when explaining the importance of the relation be-tween posterior structures and the extension gap in a cadaver study.16 Sugama et al. reported in their opera-tive study that bone cut from the posterior femoral condyles could change the tension of the posterior soft tissue structures and so alter the width and shape of the extension gap.17 These previous reports support our hypothetical mechanism.

Thus, during the TKA procedure, adjusting the soft tissue balance without a femoral component may be misleading because the femoral component insertion tightens the posterior structures, making the extension gap smaller. Furthermore, even if the femoral compo-nent were inserted to adjust the soft tissue balance, our result also suggests the importance of confi rming the fl exion angle of the knee joint precisely to evaluate the extension gap because a large change in gap value can occur with even a slight change of fl exion angle from 0° to 10° of fl exion. Thus, surgeons need to consider a more accurate gap adjustment procedure with the fem-oral component inserted at a controlled fl exion angle as a future improvement in the TKA procedure.

The ligament balance between the medial and lateral sides was almost equal from 0° to 30° of fl exion; how-ever, lateral joint laxity was signifi cantly more than me-dial from 60° to 120° of fl exion. Tokuhara et al. reported that the tibiofemoral fl exion gap at 90° of fl exion in the normal knee was not rectangular, and lateral joint laxity was signifi cantly more than medial in their in vivo study using magnetic resonance imaging estimated by the application of varus-valgus stress.18 The present study supports their report and is better suited to the TKA procedure with the use of controlled joint distraction force.

Medial joint stability was almost consistent from 10° to 120° of fl exion; however, lateral laxity became gradu-ally increased with fl exion. It seems that the medial pivot motion of the normal knee joint can be explained by this result.

The limitation in this study was that we did not ex-amine osteoarthritic knees but normal knees. There-

Table 2. Calculated values throughout the range of knee fl exion

Parameter 0° 10° 30° 60° 90° 120°

Gap based on the tibial osteotomized surface (mm) Medial 8.4 ± 1.4 11.0 ± 1.0 11.2 ± 1.3 10.6 ± 1.7 10.6 ± 1.7 10.7 ± 1.8 Lateral 12.8 ± 0.8 17.8 ± 1.4 19.2 ± 2.5 21.0 ± 2.6 21.8 ± 2.5 20.1 ± 1.7

Joint gap (mm) Medial 4.0 ± 1.5 6.5 ± 1.1 6.8 ± 1.5 6.2 ± 1.6 6.2 ± 2.0 6.3 ± 2.1 Lateral 2.8 ± 1.2 7.8 ± 1.7 9.2 ± 2.7 11.0 ± 2.9 11.8 ± 3.1 10.1 ± 2.3

Fig. 3. Average medial and lateral joint gap with a distraction force during knee fl exion. *P < 0.01 vs. another angles. **P < 0.05 between the medial and lateral gaps

K. Tanaka et al.: Soft tissue balance for CR TKA 153

fore, some pathological differences may exist between osteoarthritic and normal knees, such as the properties of the ligament. Regardless of this limitation, the results of this study could indicate the relation of the medial and lateral stabilities through the range of motion of the normal knee, which can be baseline data for reconstruc-tive surgery of the knee joint.

Conclusions

Our in vitro study using ACL-resected normal cadaver knees showed that the joint gap at 0° of fl exion was signifi cantly smaller than at other fl exion angles, and that lateral joint laxity was signifi cantly more than me-dial from 60° to 120° degrees of fl exion. These physio-logical characteristics of soft tissue balance by distraction force should be extrapolated when considering the joint status for cruciate-retaining TKA.

Acknowledgments. The authors would like to thank Kouki Nagamune, PhD (Department of Orthopedic Surgery, Kobe University Graduate School of Medicine) for collecting and analyzing data. We also acknowledge Mrs. Janina Tubby for her assistance in preparation of this manuscript.

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