MATERIALS AND METHODS

A rig was constructed into w hich the subjects were positioned fo r testing (Fig 1 ). The frame­ w ork was made from nonmetallic materials to avoid interfering w ith the magnetic fields o f the motion measurement system, w hich was a Pol- hemus 3-Space Tracker (Polhemus, Colchester. V T). The subject stood in the rig w ith the back o f the thighs against adjustable shaped pads made from rig id foam, such that the knee was at 20° flexion. A broad strap held the waist in place to maintain the subject in a vertical posi­ tion. A horizontal bar was locked tig h tly across the front o f the knee w ith a shaped pad to engage the patella. The source o f the Space Tracker was affixed to this bar. A n orthotic fixture, on the front o f which was the sensor o f the Space

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January, 1995 AP Stability in Total Knee Arthroplasty 8 9

Fig 1. A s chem atic of the test rig show s th e leg c la m p ed into the fram e and the location of the source and senso r for m easuring the m o­ tion. -Source ■Sensor Electronic Unit Computer FO RCE i— Load ceil

Tracker, was strapped along the anterior tibia. The foot was ti.xed to a load platform interfaced w ith a computer to record the foot-to-ground force during testing. A strap arrangement w ith a handle in front was fixed around the top o f the tibia. The handle incorporated a load cell that interfaced w ith the computer to record the AP force. Software was w ritten to continuously re­ cord the relative motion between the femur and the tibia at 60 Hz. and to indicate when the .AP forces reached their required maxima.

W ith the subject in the ng and the fixtures as tight as could be tolerated, equal weight was placed on each leg. which was checked by the readings from the force plate. It was found that this was readily accomplished with only small fluctuations in the foot-to-ground force. .An ante­ rio r force o f 100 N was applied to the tibia in 2 to 3 seconds, the force was relaxed to zero, and then a posterior force o f 100 N was applied. This was repeated 3 times to enable the subject to become fa m ilia r w ith the procedure and to pre­ condition the knee. The data were recorded from the next force application. The subject then leaned over onto the opposite leg to relieve weight from the test leg. The .AP draw test was repeated.

Although a higher .AP force than 100 N might have been ideal to compare with the forces cal­ culated in a c t i v i t i e s . m a n y subjects could not tolerate a higher force. To examine the reproduc­ ib ility o f the method. 2 subjects were tested 4 times, going through the entire procedure each time, including being fixed in the rig. For the

first subject, the .AP displacements were in the range o f 4.7 to 5.2 mm. and fo r the second sub­ ject they ranged from 5.1 to 5.9 mm. This was considered sufficiently reproducible fo r this study.

To assess the rig id ity o f fixation o f the subject in the rig. motions other than .AP displacement were recorded for all tests. The maximum values were 2° flexion. 1° internal-external rotation, and 0.5° varus-valgus rotation. This indicated secure fixation, as well as defining the test as constraining the other motions other than .AP displacement.

The fo llo w in g subjects were tested; 10 knees in 7 normal subjects, but 2 knees had known anterior cruciate deficiency; 15 knees in 15 sub­ jects who had Kinemax Condylar implants (pos­ terior cruciate retaining) (Howmedica. Ruther­ ford. NJ); 10 knees in 10 subjects w ith Press F it Condylar implants (posterior cruciate sacrific­ ing) (Johnson and Johnson. Braintree. M A ); and

12 knees in 9 subjects w ith In sa ll-B u rs te in Pos­ terior Stabilized implants (posterior cruciate sac­ rific in g ) (Zim m er. Warsaw. IN ). The criteria fo r selection o f the subjects were postoperative time o f 1 to 3 years and good to e.xcellent clinical results as determined from the Hospital fo r Spe­ cial Surgery sconng system. Normal alignments o f the components in frontal and sagittal planes were confirmed from the radiographs. The sub­ jects were 55 to 85 years old.

Tests then were performed on samples o f the prosthetic components themselves, provided by the manufacturers from standard stock. The

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Clinical Orthopaedics and Related Research

components were mounted onto plastic bones and affixed in the test rig at 20° flexion, the same angle as for the test subjects. A platform was located at the level o f the femoral head on which weights could be placed along a vertical axis through the center o f the knee. A n te rio r-p o ste ­ rio r tests were performed under exactly the same conditions as for the subjects, w ith the weight progressively increased from 200 N to 700 N. To measure the frictional contribution o f resis­ tance to the AP force, the tests were performed under dry conditions, w ith distilled water, bo­ vine serum, and bovine synovial fluid. The sur­ faces were thoroughly cleaned between the ap­ plication o f each lubricant. R eproducibility tests were performed fo r each o f the 3 designs by performing the tests 20 times under dry condi­ tions. There was a high reproducibility with a maximum range o f 1 mm and a standard devia­ tion < 0.3 mm.

The radii o f curvature o f the prosthetic sur­ faces in the sagittal plane was measured using a calibrated traveling microscope. .An analysis was then developed to theoretically predict the AP motion under the conditions o f the test, using the radii data. H aving established the relation between the theory and experiment, the theory was extrapolated to predict the AP displace­ ments under conditions o f higher load beanng to simulate functional situations.

R E SU L T S