Development and Validation of a Non-invasive Tracking System for Measuring 3-D Dynamic Knee Laxity In VivoBoddu Siva Rama KR1, Cuomo P2, Bull AMJ3, Thomas RW4, Amis AA5

1,2,5 Department of Mechanical Engineering, 3 Department of Bioengineering, and 1,4 Institute of Musculoskeletal SurgeryImperial College, London, SW7 2BX, UK.

INTRODUCTION

In vivo knee laxity measurement devices have been traditionally limited to a single plane measuring either sagittal translations (KT arthrometer) or axial rotations (rottometer) with limited accuracy. The aim of this study was to develop a non-invasive portable system to measure the three dimensional laxity of knee joint in vivo in real time and to validate its accuracy.

• The femoral attachment device consists of a clamp component to hold the femoral condyles and a long arm strapped to the anterior aspect of thigh (Fig 1). A pivot joins these two components so that the movements of the soft tissues of thigh relative to the femoral condyles can be decoupled.

• The tibial attachment device is a flat rectangular block that can be strapped firmly over the anteromedial subcutaneous surface of tibia (Fig 2).

• This system was validated in anterior cruciate deficient knees of nine young adults under anaesthetic during their cruciate reconstruction operations following the same protocol described above.

RESULTS The femoral splint showed root mean square (RMS) errors of 4.3 mm for AP translations (Fig 3) and 9.6° for axial rotations. The tibial splint showed RMS errors of 2.5 mm for AP translations and 4.0° for axial rotations. The femoral attachment device showed RMS errors of 1.8 mm in measuring the AP translations of 11.7 ± 2.8 mm and 3.6° in measuring the axial rotations of 24.8° ± 5.6. Maximal RMS error (5.3°) was found for the internal rotation movement at extension under internal torque and for the rest of the loading conditions the RMS error for axial rotations was 1.3°. The tibial attachment device showed RMS errors of 0.5 mm and 1.6° for AP translations and rotations respectively (Fig 4).

MATERIALS AND METHODS• Electromagnetic motion tracking system (Nest of Birds,

Ascension Technology Co., Vermont) with a known accuracy1 of ± 0.3 mm for measuring 100 mm translation and ± 0.6° for measuring 30° rotation were used.

• Initially moulded thigh and shin splints were used to attach a sensor each to the thigh and shin. A third sensor with stylus tip was used to digitise the femoral epicondyles.

• The centre of the hip was digitised by circumductory movements of hip joint.

• The joint co-ordinates to define the knee motion with six degrees of freedom were developed (Grood and Suntay convention2) from these three bony landmarks while holding the knee in a ‘neutral’ extension position.

• The accuracy of this system was validated in nine full length cadaveric lower limbs.

• Two additional sensors were fixed to the femur and tibia using intracortical pins (2 mm K wires).

• Translational and rotational loads were applied at different flexion angles.

• The knee motion was measured simultaneously from both the bone fixed sensors and from the splint mounted sensors.

• The differences between the external sensor measurements and the bone fixed sensor measurements were calculated.

• Because of poor results of these splints, special devices were developed subsequently to improve the attachment of the sensors to the femur and tibia.

ACKNOWLEDGEMENTS

REFERENCES

1.Bull et al Proc Inst Mech Eng [H]. 1998;212(5):347-55.

2.Grood & Suntay J Biomech Eng. 1983;105(2):136-44.

Sensor fixed to tibia

Sensor fixed to femoral attachment device

Sensor fixed to femur

Transmitter of “Nest of Birds”

CONCLUSION This non-invasive tracking system using special attachment devices can measure knee laxity in vivo in three dimensions with reasonable accuracy. Further improvements in the design of the attachment devices are in progress to increase the accuracy

Figure 1. Validating the accuracy of the femoral attachment device for tracking the femoral movements.

Figure-2. Validating the accuracy of the tibial attachment device for tracking the tibial movements.

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Sensor fixed to tibial attachment device

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Figure 3. Errors in measuring antero-posterior laxity with splints and special attachment devices

Figure 3. Errors in measuring axial rotational laxity with special attachment devices

This work was supported by grants from Arthritis Research Campaign and Hammersmith Hospitals Research Trustee Committee