CHANGES IN LENGTHS OF ANTERIOR CRUCIATE LIGAMENT FIBRES DURING TIBIAL TRANSLATION AT THE KNEE

Ahmed Imran

Ajman University of Science and Technology, Ajman, UAE.

Introduction

At the knee joint, anterior cruciate ligament (ACL)

is the primary restrain to anterior tibial translation

(ATT) relative to femur. Experimental studies have

recorded the variation of ATT due to applied

anterior loads on tibia at selected flexion angles

[Lo, 2011]. Depending on the relative positions of

the two bones, some fibres in the ACL can stretch

while others can slacken. Therefore, it is important

to study the state of different fibres of the ligament

during any specific motion.

In the present study a mathematical model of the

knee is used to analyze the patterns of ACL fibre

length changes resulting from ATT at several

flexion angles of the joint.

Methods

The knee was modelled in the sagittal plane with

cruciate and collateral ligaments represented as

bundles of non-linear elastic fibres. The articular

surfaces were assumed to be frictionless and

impenetrable. Passive motion of the joint was

defined during 0–120o flexion such that selected

fibres in the cruciate ligaments remained isometric.

Anatomical data and material properties of the

ligaments were taken from literature [Zavatsky,

1992]

An anterior laxity test was simulated at selected

flexion positions by applying 130N anterior force

on tibia along with a balancing moment to maintain

joint angle. Distance between bony attachments of

a ligament fibre gave its length. In comparison to

reference lengths defined at 0o flexion, a fibre

stretched if its length increased and slackened if its

length decreased. During passive flexion and with

ATT, changes in the fibre lengths were calculated

for three fibres as percentage of their respective

reference lengths – anterior (dLa), intermediate

(dLi) and posterior (dLp).

Results and Analysis

Figure 1 shows results of the simulated laxity test

and comparison with mean values from

experimental observations on intact cadaver knees

[Lo, 2011]. The ATT first increased for 0–45o range

and then decreased in higher flexion. The

calculations agree reasoably with the experiment.

Figure 2 shows the calculated % change in lengths,

dLa, dLi and dLp, for the three ligament fibres.

During passive motion, the anterior fibre remained

just taut, while the intermediate and posterior fibres

were slack. Further, with ATT due to 130N test, the

anterior fibre remained stretched for all flexion

angles; the middle fibre stretched till 60o and then

at 120o; The posterior fibre stretched only at 120o.

In vitro experimental observations during passive

flexion show that the antero-medial bundle of the

ACL was first slack and then tight in flexion, while

the intermediate and postero-lateral bundles

remained slack relative to 0o flexion [Amis, 1991].

Figure 1: Anterior tibial translation (ATT) over the

flexion range. ‘×’ show standard deviation.

Figure 2: ACL fibre length changes (%) calculated

during passive flexion (dashed lines) and with ATT

due to 130N laxity test (continuous lines).

Conclusions

Lengths of the ACL fibres showed variation with

flexion angle and with tibial translation. Such

changes suggest that the ligament injuries can affect

different fibres depending on the joint position at

the time of injury. The model results agree

qualitatively with the experimental observations.

References

Amis et al, J Bone Jt. Surg. 73B(2): 260–267, 1991.

Lo JH et al, The Knee, 18(6):491–495, 2011.

Zavatsky et al, J Engg in Med. 206: 125–134, 1992.

Presentation 1192 − Topic 29. Knee biomechanics S373

ESB2012: 18th Congress of the European Society of Biomechanics Journal of Biomechanics 45(S1)