Contribution of the Meniscofemoral Ligament as a Restraint to the Posterior Tibial Translation in a Porcine Knee
+1,2Lertwanich, P; 1Martins, CAQ; 1Kato, Y; 1Ingham, SJM; 1Kramer, S; 1Linde-Rosen, M; 1Smolinski, P; 1Fu, FH.
+1University of Pittsburgh, Pittsburgh, PA; 2Mahidol University, Bangkok, Thailand
[email protected]
INTRODUCTION:
The meniscofemoral ligament (MFL) is a major structure in the
posterior aspect of the porcine knee together with the posterior cruciate
ligament (PCL). The MFL attaches from the posterior horn of the lateral
meniscus to the medial aspect of the intercondylar area of the femur.
Since the MFL is located posterior to the PCL, it is comparable to the
posterior meniscofemoral ligament (Wrisberg’s ligament) in the human
knee.(1) The porcine knee is a frequently used animal model for
biomechanical evaluation of the PCL reconstruction techniques. The
contribution of the MFL to knee stability in this animal model is not well
understood. A better understanding the biomechanical function of this
ligament will further facilitate PCL studies using the porcine model.
The purposes of this study are 1) to evaluate the kinematics of the
knee after sequential transection of the PCL and MFL, and 2) to
determine the in situ forces of the PCL and MFL in response to posterior
tibial load. We hypothesize that the MFL is the secondary restraint to the
posterior tibial translation (PTT) in conjunction with the PCL.
METHODS:
Ten fresh-frozen hind limbs from six month old pigs, a cross between
Yorkshire and Hamp, were used in this study. The femur and the tibia
were cut approximately 15 cm from the joint line and potted in
cylindrical shaped epoxy compounds (Bondo, Atlanta, GA) for
mounting in custom-made aluminum fixtures. The femoral fixture was
securely attached to the base of the robotic manipulator, while the tibial
fixture was connected, through the universal force-moment sensor
(UFS), to the end-effector of the robot (Fig 1). Prior to the test, muscles
at the posterior knee were removed to expose the posterior capsule.
Then, a posterior capsular window was made between the inner border
of both femoral condyles to allow visualization from the femoral
insertion of the MFL proximally to the tibial insertion of the PCL
distally to be able to identify the MFL and PCL (Fig 2).
Fig 1 Porcine knee on the robotic/ UFS testing system
Fig 2 Posterior capsular window
The robotic manipulator (CASPAR, OrthoMaquet, Rastatt, Germany)
is a six-joint, serial articulation device which is capable of achieving
positional control of the joint in six-degree-of-freedom. The UFS (model
4015, JR3 Inc, Woodland, California) has the ability to measure three
orthogonal forces and moments simultaneously with repeatability of 0.2
N and 0.01 Nm respectively.
The path of passive flexion-extension of the intact knee from 15⁰ to
90⁰ of flexion was firstly determined by the robotic/ UFS testing system.
The robotic manipulator moved the tibia by 1⁰ increments of flexion
while the forces and moments were minimized by an iterative algorithm.
The knee was then preconditioned with a 89 N posterior tibial load for 5
times. The testing protocol is demonstrated in Fig 3.
Statistical analysis for differences in PTT at each flexion angle was
performed using Friedman test, following by post hoc analysis to
compare differences between each pair of three knee conditions using
Wilcoxon signed ranks test.
RESULTS:
The magnitude of PTT was sequentially increased, from the intact
condition, after cutting the PCL and the MFL as demonstrated in Fig. 4.
The difference in PTT between the intact knee and the PCL deficient
knee under 89 N posterior tibial load was statistically significant at every
flexion angle from 15⁰ to 90⁰ (p<0.05). The difference of PTT was
greatest at 90⁰ of flexion (13.23 ± 2.17 mm), and smallest at 15⁰ of
flexion (4.90 ± 2.50 mm). The difference in PTT between the PCL
deficient knee and the PCL and MFL deficient knee was also statistically
significant at each testing angle (p<0.05). The increment in PTT after
cutting the MFL was highest at 15⁰ of flexion (2.68 ± 0.94 mm), and
lowest at 90⁰ of flexion (1.13 ± 0.65 mm).
The highest in situ force of the PCL was measured at 60⁰ of flexion
(82.25 ± 8.59 N), and the lowest was found at 15⁰ of flexion (45.08 ±
15.86 N) as shown in Fig. 5. The in situ force of the MFL was highest at
15⁰ of flexion (24.29 ± 6.45 N), and was lowest at 90⁰ of flexion (12.91
± 10.49 N). Regarding the ratio between the in situ force of the MFL
and that of the PCL, the number was highest at 15⁰ of flexion (53.89 %),
and was lowest at 75⁰ of flexion (16.20 %).
Fig 4 Posterior tibial translation
of the knees in each flexion
angle (* significant different
from the intact knee, p<0.05; †
significant different form the
PCL deficient knee, p<0.05)
Fig 5 In situ force of the PCL
and MFL in response to the
posterior tibial load at
different flexion angles
DISCUSSION:
The findings in this study revealed the contribution of the MFL in
resisting the PTT in addition to the PCL, especially near full extension.
The in situ force of the MFL was highest at 15⁰ of flexion, and the
increment of PTT after cutting this ligament was also highest at the same
flexion angle. This was in contrast to the PCL that had the lowest in situ
force at 15⁰ of flexion. The tension pattern of the porcine MFL was
comparable to the posterior meniscofemoral ligament in the human
knee, described in a previous study as which the previous study
mentioned that it was being tense near full extension.(4)
In this study, a posterior capsular window was utilized to identify the
PCL and the MFL that could increase PTT. However, this effect was
neutralized for in situ force calculation by creating the capsular window
in the intact knee and maintained in the same size throughout the test.
ACKNOWLEDGEMENT:
Fig 3 Testing protocol and data obtained
A posterior tibial load of 89 N was applied to the knee at 15⁰, 30⁰,
45⁰, 60⁰, 75⁰, and 90⁰ of knee flexion. This load was used to simulate
the posterior drawer test in which the PCL served serves as a primary
restraint. The principle of superposition was used to calculate the in situ
forces of the PCL and MFL in response to posterior tibial load at
different flexion angles.(2,3)
The support of the Albert B. Ferguson, Jr. MD Orthopaedic Fund of The
Pittsburgh Foundation is gratefully acknowledged.
REFERENCE:
(1) Gupte CM, et al. Anat Histol Embryol. 2007. (2) Rudy TW, et al. J
Biomech. 1996. (3) Woo SL, el al. J Bone Joint Surg Am. 2009. (4)
Gupte CM, et al. J Bone Joint Surg Br. 2003.
Poster No. 1152 • 56th Annual Meeting of the Orthopaedic Research Society