Testing Technique

Setting Up for PCL Tests

Setting Up for ACL Tests

1.  Relax the Patient. The patient should lie supine, with his hands at his side, on a comfortable, firm examining table. The table should be long enough to support the patient's head and heels; the patient's head should be comfortable, supported with the cushion of the table or a pillow, his hands over his midsection and his eyes looking to the ceiling. You may wish to use a modesty drape. (See checklist )
2.   Thigh Support Platform. Place thigh support platform under both legs at a level proximal to the popliteal space. This platform will keep the flexion angle of both knees symmetrical during testing. The thigh support platform is adjustable, raising the thigh an additional 6.5 cm (earlier production models are not adjustable; place a board or book under the platform to alter elevation).  Elevating the thigh support will increase the flexion angle. It is important to obtain an angle of flexion which well establishes the patella in the trochlea. Patellar mobility will produce test error.
3.  Foot Support Platform. Place the foot support platform under both feet of the patient at a level distal to the lateral malleolus. The patient's heels should rest comfortably on the foot pad immediately adjacent to lateral upright ends. Note: Some patients must be tested with the heel resting on the foot pad at a distance in from the lateral uprights to prevent excessive internal rotation of the knee. In these cases, draw an imaginary line from the medial border of the foot down to the ruler on the foot pad. Read the actual distance that both heels are from the upright on the foot support's centimeter
scale; record readings on the Patient Evaluation Form.

 4.  Check Flexion Angle. Measure the angle of flexion using the goniometer; if necessary, adjust thigh support platform so that knee is between 20º and 35º of flexion. The knee must be flexed sufficiently for the patella to engage the femoral trochlea. Record the flexion angle on the Patient Evaluation Form.
 5.  Place Thigh Strap. Use the Velcro thigh strap if there is an excess of hip external rotation (either naturally or in the operating room when the patient is under general anesthesia). Place strap so it resists external rotation on both legs equally, then tighten until it removes much of the internal rotation torque on each knee when feet are in the foot support.
 6.  Position the ARTHROMETER®. Secure the Knee Ligament ARTHROMETER^® on the anterior aspect of the tibia so that the joint-line arrow aligns with the joint line of the knee; apply the distal Velcro strap (without over tightening, which may cause discomfort and interfere with relaxation). Rotate the instrument so that pressure on the patella pad stabilizes the patella. If the patella is unstable, elevate the thigh support further to flex the knee and bring the patella down into the femoral trochlea. Firmly secure the proximal Velcro strap.
 7.  Adjust ARTHROMETER®. Confirm that the joint line arrow is at the level of the joint line and that pressure on the patella stabilizes the patella in the femoral trochlea. To adjust the ARTHROMETER® for patella height, loosen the patella sensor adjustment knob and position the tibia and patella rails nearly parallel with one another; tighten the knob. 
8.  Manually Oscillate the Calf. Grasp the back of the patient's proximal calf and gently but rapidly oscillate the tibia in an anterior and posterior cycle several times; this will induce a relaxation response in the muscles. When the ARTHROMETER® displacement needle moves freely, the patient is relaxed.
 9.  Stabilize the Patella. Place one hand on the thigh and the patella reference pad to stabilize the instrument. Push posteriorly on the patella reference pad until there is no further movement on the displacement dial. Maintain this pressure throughout the examination.
10.  Position Tibia and Condition Tissue. Pull and then push on the force handle (this will produce three anterior audio tones when pulling and posterior tones when pushing); when the posterior tone sounds, release the force. Repeat. This posterior push and release maneuver is performed two or three times (until the displacement dial returns to the same position).
11.  Adjust Dial. While maintaining constant pressure on the patella reference pad, rotate the displacement dial to align the needle with zero.

The Testing Reference Position

The testing reference position is the zero position from which all other measurements, posterior and anterior, will be made.  Failure to establish a repeatable testing reference testing position may indicate a lock of patient cooperation (tense thighs, legs held together or rigid, tight patellar tendon during manual palpitation).

Patient Relaxation Checklist

Other Considerations

Setting Up the X-Y Plotter (KT2000 Only)

A - Power
B - Off-Chart-Pen Switch
C - "X" Adjustment Knob
D - "Y" Adjustment Knob
E - Pen
 
 
 
 
 

1. Connect to ARTHROMETER^®. Connect coiled cord to plotter's five pin, circular connector and to the rear jack of the ARTHROMETER^®.

2. Insert Patient Evaluation Form.

a. Move plotter's power switch (the red switch on the lower left corner on top of the plotter) to the "1" position.

b. Place a KT2000 Patient Evaluation Form on the plotting surface, grid side up with the lower edge of the form against the surface edge.

c. Move the Off-Chart-Pen switch to the "Chart" position. Make sure paper is on the plotting surface before switch is moved to "Chart;" retention force of the surface will be reduced if the plotter is on "Chart" without paper on this surface.

3. Drop the Plotter Pen. Depress the switch on either side of the Knee Ligament ARTHROMETER^® force handle; the pen will remain in position as long as the switch is held.

4. Zero the Plotter. Push on the force handle until you hear the 20 lb. tone; release the force handle. The pen should return to the 0 force line on the graph paper. (If it does not, move the plotter pen up or down on the paper using the vertical adjustment knob on the upper right of the plotter until the pen drops directly onto the 0 force line). After the initial posterior push, you will have only 40 seconds to zero the plotter (the ARTHROMETER^® turns itself off automatically). If necessary, give the unit another push to re-start it.

5. Place Pen in "Test" Position. Using the horizontal adjustment knob, place the pen in the desired position to begin the force/displacement curve. Each graph form has ample room for testing and comparing left and right knees. Up to three curves on each knee (six total readings) may be recorded on a single page.
 

Setting Up the X-Y Plotter (Recent Model KT2000)

A - Power Pen Up/Down Lever
B - Paper Clamp
C - Pen
D - "X" Adjustment Knob
E - "Y" Adjustment Knob
 
 
 
 
 
 
 
 
 

1. Connect to ARTHROMETER^®. Connect coiled cord to plotter's five pin, circular connector and to the rear jack of the ARTHROMETER^®.

2. Insert Patient Evaluation Form.

a. Move plotter's power switch (the red switch on the lower left corner on top of the plotter) to the "load"
position. This moves the pen-holder to the upper right corner and releases the right-hand paper clamp.

b. Release the left side paper clamp by depressing the tabs. Remove the stub or cardboard backing
remaining from the previous pad.

c. Lay the new pad on the surface and slide it well under the right side paper clamp. Depress the left-
side paper clamp and slide the new pad under it from right to left. The pad should line up against the
left paper stop pin (under the left side clamp) and the bottom edge of the paper stop. Release the left-
side clamp. Smooth pad against surface by moving your hand from left to right over the pad.

3. Drop the Plotter Pen. Move the plotter's operating switch to the Pen Down position.

4. Zero the Plotter. Push on the force handle until you hear the 20 lb. tone; release handle. The pen should return to the 0 force line on the graph paper. (If it does not, move the plotter pen up or down on the paper using the vertical adjustment knob on the upper right of the plotter until the pen drops directly onto the
0 force line). After the initial posterior push, you will have only 40 seconds to zero the plotter (the ARTHROMETER^® turns itself off automatically). If necessary, give the unit another push to re-start it.

5. Place Pen in "Test" Position. Using the horizontal adjustment knob, place the pen in the desired position to begin the force/displacement curve. Each graph form has ample room for testing and comparing left and right knees. Up to three curves on each knee (six total readings) may be recorded on a single page.

Pull-Push testing Cycle (KT1000)

2. Establish the Testing Reference Position.

3. Maintain Pressure on the Patella Pad throughout Cycle.

4. Pull: smoothly and slowly through the force handle (with the pull in line with the handle shaft); tones will sound at 15 lbs., 20 lbs. and 30 lbs. of force. Do not exceed 30 lbs. of force.

Note: Excessive application of force beyond the 30 lb. anterior and 20 lb. posterior tones will result in patient guarding due to discomfort at contact points on the tibia and possible pain from excessive force.

5. Push: on the force handle until the 20 lb. posterior tone sounds. Do not exceed 20 lbs. of force.

6. Release Force Handle. The dial should return to "0", ± 0.5 mm. This indicates the orientation of the instrument is unchanged and the patient has remained relaxed.

7. Record Readings. Record displacement readings at each tone on the Patient Evaluation Form.

8. Repeat (If Necessary). If the dial does not return to within 0.5 mm of zero, do not use measurements; re-establish testing reference position and repeat cycle.
 

Pull-Push Testing Cycle (KT2000)

1. Set up Patient for ACL or PCL Tests.

2. Establish the Testing Reference Position.

3. Maintain Pressure on the Patella Pad throughout Cycle.

4. Zero Plotter. Adjust X-Y plotter to zero.

5. Pull: while depressing the pen button (in the force handle),pull smoothly and slowly through the force handle (with the pull in line with the handle shaft); tones will sound at 15 lbs., 20 lbs. and 30 lbs. of force. Do not exceed 30 lbs. of force.

6. Push: on the force handle until the 20 lb. posterior tone sounds. Do not exceed 20 lbs. of force.

Note: Excessive application of force beyond the 30 lb. anterior and 20 lb. posterior tones will result in patient guarding due to discomfort at contact points on the tibia and possible pain from excessive force.

7. Release Force Handle. While maintaining pressure on the pen button, release posterior force; the dial should return to "0", ± 0.5 mm. Remove pressure from the pen button. (See illustration).

8. Repeat (If Necessary). If the pen does not return to within 0.5 mm of zero, do not use measurements; re-establish testing reference position and repeat cycle.

9. Measure. Place the clear, plastic reticular scale on the plotter graph. If the scale is missing, count vertical lines (from the zero reference point); each line equals 1 mm. Displacement to the left of zero (or the starting reference position) indicates posterior displacement; displacement to the right indicates anterior displacement.
 
 

Push-Pull Testing Cycle (KT2000)

Note: The KT2000™ has a specialized battery saving feature which automatically shuts off the ARTHROMETER^® battery power after 40 seconds of inactivity. To re-start the ARTHROMETER^® push on the force handle until you hear the posterior force tone; then continue with your tests.

1. Set up Patient for ACL or PCL Tests 

2. Establish the Testing Reference Position.

3. Maintain Pressure on the Patella Pad throughout Cycle.
4. Zero Plotter. Adjust X-Y plotter to zero.

5. Push. Push on the force tone handle until the posterior 20 lb. tone sounds. Depress button on force handle. Do not exceed 20 lbs. of force.

6. Pull. Pull forward on the force tone handle until the 30 lb. anterior tone sounds; release button to lift the plotter pen. Do not exceed 30 lbs. of force.

7. Push. Push posteriorly again until the 20 lb. posterior tone is heard; release the force handle while continuing to stabilize patella. The instrument should return to zero ± 0.5 mm; this confirms that the instrument testing reference position has not changed.

Note: Excessive application of force beyond the 30 lb. anterior and 20 lb. posterior tones will result in patient guarding due to discomfort at contact points on the tibia and possible pain from excessive force.

8. Measure. Place the clear, plastic reticular scale on the plotter graph. If the scale is missing, count vertical lines (from the zero reference point); each line equals 1 mm. Displacement to the left of zero (or the starting reference position) indicates posterior displacement; displacement to the right indicates anterior displacement.

90 Degree Quadriceps Active Test (Visual)

1. Position Patient. Have patient lie supine on examining table. Have patient flex knee, keeping your eyes at the level of the flexed knee; rest one elbow on the table, using your hand to support the patient's thigh and to confirm that thigh muscles are relaxed. See Testing Reference Position and checklist.

2. Apply Force. Using the other hand, apply a brief posterior force to the tibia and then lightly restrain the patient's foot. Ask patient, "gently slide your foot down the table;" your hand on the foot will prevent the leg from sliding. Anterior tibial displacement resulting from the quadriceps contraction indicates that the PCL is disrupted. 

3. Quantify 90° Quadriceps Active Findings: This is a more sensitive test to screen for PCL disruption.

a. Set up Patient for PCL Testing.

b. Establish Testing Reference Position.

c. Test the Non-Involved Knee. Ask patient to "Gently try to slide your foot down the table;" note displacement on the dial. Apply a momentary 20 lb. posterior push on the force handle. The dial needle should return to zero ± 0.5 mm.

e. Interpreting Findings. Anterior movement of the tibia indicates an injury to the posterior cruciate ligament; posterior movement from 0.5 mm to 2 mm of the tibia indicates a normal knee.

4. Follow-up Testing. If the tibia moves anterior during quadriceps active testing, perform further PCL tests at the quadriceps neutral angle. If quad active tests do not indicate a PCL, perform ACL tests.
 
 

Tests at the Quadriceps Neutral Angle

1. Set up Patient for PCL Testing.

2. Establish Testing Reference Position.

3. Establish the Quadriceps Neutral Angle on the Non-Involved Knee. Ask patient to "Gently try to slide your foot down the table" (while restraining the foot). Both the position of the foot and the flexion angle of the knee should remain constant. If there is more than 1 mm of deflection on the dial, adjust the knee flexion angle and repeat until the flexion angle is identified that has no more than 1 mm of tibial motion with the quadriceps active test. This is the quadriceps neutral angle (in studies, this angle has ranged from 55° to 85°). Measure the angle of the knee with the goniometer and record the quadriceps neutral angle on the Patient Evaluation Form.

4. Perform the Passive Displacement Test on the Non-Involved Knee. Perform the standard pull-push testing cycle. Record testing angle, 20 lb. anterior and 20 lb. posterior displacements on the Patient Evaluation Form.

5. Perform the Quadriceps Active Test on the Non-Involved Knee. Zero the ARTHROMETER^® and have the patient slide the restrained foot forward; record the result on Patient Evaluation Form. The dial should read zero (at QNA).

6. Perform the Passive Displacement Test of the Involved Knee. Perform the standard pull-push testing cycle. Record testing angle, 20 lb. anterior and 20 lb. posterior displacements on the Patient Evaluation Form.

7. Perform Quadriceps Active Test on the Involved Knee. Zero the ARTHROMETER^® and have the patient slide the restrained foot forward; record result on Patient Evaluation Form.

8. Adjust Measurements. Normalize displacements to the quadriceps active position. The corrected anterior displacement is displacement anterior to the quadriceps active position (measured anterior displacement minus the quadriceps active displacement). The corrected posterior displacement is displacement posterior to the quadriceps active position (measured posterior displacement plus the quadriceps active displacement).

10. Interpret Data. A difference of 2 mm or greater usually indicates pathology.

11. Follow-up Testing. Proceed to ACL tests. Note: Only the compliance index passive displacement test may be used in the case of the PCL disrupted knee.

For further information on instrumented PCL testing, refer to the video program, Instrumented Drawer Testing, or to the booklets, The Quadriceps Active Test, and, The Active Drawer Test. All are included with the KT1000™ and KT2000™; additional copies are available directly from MEDmetric^® Corporation.
 
 

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Quadriceps neutral angle test measurements and adjustments as recorded on the Patient Evaluation Form (above) and the KT2000 X-Y plotter. Note that in this case the quad neutral angle is at 70° of flexion; studies have indicated quadriceps neutral angles ranging from 55° to 85°.
 
 
 
 
 

Passive Displacement Test

1. Set up Patient for ACL Testing.

2. Establish the Testing Reference Position.

3. Test the Non-Involved Knee. Mark the medial joint line; perform the standard pull-push test.  Record measurements at 15 lbs., 20 lbs. and 30 lbs. of anterior force on the Patient Evaluation Form.

4. Test the Involved Knee. Support the involved knee at the non-involved knee test flexion angle; establish the testing reference position for 30º. Perform standard pull-push test; record 15 lb., 20 lb. and 30 lb. anterior displacements.

5. Interpreting Data. In most individuals, the measured difference between knees is usually less than 3 mm.

 

The results of 30° passive displacement tests as recorded on the Patient Evaluation Form (above) and the X-Y plotter grid (left). The graph on the left describes the non-involved knee; the one on the right the involved knee.

6. Possible Inconsistencies. Force vectors, rotation of the ARTHROMETER^® or parallax may create measurement inconsistencies. If measurements are inconsistent on the 30° tests, check the following:

a. Transient Muscle Guarding (Solution: Ensure patient is relaxed; manually oscillate calf to further relax muscles)

b. Variation in the Degree of Initial Tibial Rotation (Solution: Ensure test is started from same position of tibial rotation through use of the thigh strap)

c. Variation in Patella Pad Pressure (Solution: Check starting pad pressure; establish patella in trochlea)

d. Failure to "Zero" between Measurements. (Solution: Give a momentary posterior push to reset the tibia at the test reference position).

e. Inconsistent Knee Flexion Angle. (Solution: Compare angle on goniometer with previously recorded angle).

f. Skin "Bumping" Error. This may be a problem in an obese patient or one with mobile skin and soft tissue around the calf. (Solution: Slide proximal Velcro strap more distal - toward the foot).

g. General Anesthesia Effects in the O.R. (Solution: Lower foot of the O.R. table to bring flexion angle closer to 35° for both knees; this will guide the patella into the trochlear groove).
Note: See "Sources of Measurement Error"

7. Follow-up Testing. Compute the Compliance Index.

The Compliance Index

Subtract 15 lb. readings from 30 lb. readings.

The resulting measurements constitute the compliance index, the slope of the force-versus-displacement curve of the patient's knee between 15 lbs. and 30 lbs. of applied anterior force.

Proceed to the Quadriceps Active Test.
 
 

Quadriceps Active Test at 30°

Purpose: To Measure tibial displacement with a quadriceps muscle contraction

1. Set up Patient for ACL Testing.

2. Establish the Testing Reference Position

3. Test the Non-Involved Knee. Ask patient to "Gently try to raise your foot off the table." Just as heel lifts, note displacement on the dial and record on the Patient Evaluation Form. Apply a momentary 20 lb. posterior push on the force handle. The dial needle should return to zero ± 0.5 mm This test may also be performed by a restraining hand on the ankle.

4. Test the Involved Knee. Repeat the sequence in Step 3 on the involved knee.

5. Interpret Results. A right/left difference of 3 mm probably indicates a pathology.

6. Follow-up Testing. Conduct manual maximum displacement test.

Quadriceps active test at 30° results on the Patient Evaluation Form (above) and the KT2000 X-Y plotter (below)

 

Manual Maximum Displacement Test

Purpose: To obtain maximum displacement and quantify endpoints.

1. a. Set up Patient for ACL Testing.

b. Establish the Testing Reference Position.

2. Test Non-Involved Knee. Use one hand to stabilize patella; use the other to apply direct manual force to the back of the subject's calf, proximal to the instrument's Velcro straps. Read and record displacement from the dial.

3. Zero the ARTHROMETER^®. Apply posterior force through the handle and release. The dial needle should return to zero ± 0.5 mm.

4. Test the Involved Knee. Repeat the sequence in Step 2 on the involved knee.

5. Interpreting Results. The Manual Maximum Displacement Test has been the most sensitive of the various tests. A right/left displacement measurement difference of 3 mm or more is indicative of an ACL disruption.

Manual maximum displacement testing as recorded on the Patient Evaluation Form (above) and KT2000™ X-Y plotter form (below).

The Active Drawer Test

BioMechanics

In early flexion of the anterior orientation of the patella tendon results in an anterior tibal translocation force.  The anterior displacement is constrained by the ACL.
 
 

As the knee flexion increases the patella tendon orientation becomes posterior, resulting in a posterior translocation force.
 
 

In Vitro Study of Quadriceps ACL Interaction

The cadaver limb is mounted and a flexion load is applied which is resisted by the quadriceps pull.
 
 
 

The tibial femoral position is documented using visual grid system
 
 
 

The ACL is cut and the flexion angles where the tibial femoral relationship is altered is noted.
 
 
 

The normal tibial-femoral relationship is reestablished with an ACL substitute cable.

With the femur oriented horizontally, the tension in the ACL cable to maintain the tibia-femoral relationship ion an anatomic position is presented as a ratio of cable tension divided by limb load times 1000 at several positions of flexion.  The added weight was located 16 inches distal to the origin of the MCL.  Seven specimens were studied using 0 to 15 lbs. of limb load.  The linear regression line indicates the mean flexion angle where the ACL was unloaded is 51 degrees.
 

20º Active Quadriceps Drawer

Anterior tibial translocation with quadriceps contraction to lift the leg may be measured with a Knee Ligament ARTHROMETER^®. If the examiner extends the knee by lifting the foot, the proximal tibia does not move anteriorly in relation to the femur.  However, when the patient contracts the quadriceps to lift the leg, the tibia translocated anteriorly.  Read the dial as the heel lifts off the table.  The translocation in 408 normal volunteers was 4 mm.  The side to side difference in the normal population and 118 ACL deficit patients is noted below.
 
 

90º  Active Quadriceps Test

With the knee in a 90º of flexion, contraction of the quadriceps in the normal knee results in 0 to 2 mm of posterior tibial displacement.

In a PCL deficit knee there is a posterior tibial sag and quadriceps contraction brings the tibia anterior.  The Knee Ligament ARTHROMETER^® may be used to measure the 90º active quadriceps drawer.

Summary

1. There is a consistent interaction between the quadriceps muscle and the cruciate ligaments which depends on the knee flexion angle.

2.  20º Active Quadriceps Drawer.

 A quadriceps contraction with the knee at 20º of flexion displaces the tibia forward.

 The ACL constrains tibial displacement.

3. 90º Active Quadriceps Drawer.

 A quadriceps contraction in the normal knee at 90º of flexion displaces the tibia posterior.

 In a PCL deficit knee with a posterior tibia sag, the quadriceps contraction pulls the tibia anterior.

4.  The Active Drawer Tests may be measures with a Knee Ligament Arthrometer®

Prepared for the 50th Annual Meeting of the American Academy of Orthopaedic Surgeons
From the Departments of Orthopedic Surgery: Kaiser Hospital, San Diego; University of California, San Diego; Donald Sharp Rehabilitation Center, San Diego

Background
 

In the 90 degree drawer position, the tibia is supported by the PCL.	The patellar tendon is orientated posterior as it passes from the tibia to the patella in the 90 degree drawer position.

 

Disruption or elongation of the PCL results in posterior sublaxation of the tibia	The patellar tendon is orientated anterior as it passes from the posterior subluxed tibia to the patella in the 90 degree drawer position.

At 90 degrees of Flexion, the Effect of a Quadriceps Contraction
 

Resting Position

tibia moves  posterior

Posterior Cruciate Ligament Injured Knee
 

Resting position

tibia moves anterior

The 90 degree quadriceps active test to diagnose posterior cruciate ligament disruption.

1. Place knee in the 90 degree drawer position.

2. The examiner sits beside the examining table with her/hiss eyes at knee level.

3. The examiner rests her/his elbow on the table and supports the patients thigh.   The examiner's hand monitors thigh muscle tone.

4. The foot is stabilized by the examiners other hand.

5. The patient is requested to "gently try to slide your foot down the table."

6. Thigh muscle tone is monitored to confirm that the quadriceps muscles contract and the hamstring muscles remain relaxed.

7. The posterior cruciate ligament injured knee will move anterior. The normal knee will move posterior 1 to 2 millimeters.

At the angle of flexion in the normal knee where the patellar tendon is normal to the joint surface, tension in the patellar tendon is normal to the joint surface, tension in the patellar tendon does not result in anterior or posterior tibial translation
 

Sectioning either cruciate ligament in this position will not result in a change in joint position in the quadriceps stabilized knee.  The position of the knee with a quadriceps contraction is termed the Quadriceps Active Position. The quadriceps active position at the quadriceps neutral angle will be altered by changes in the extensor mechanism anatomy (e.g. patellectomy or tibial tubercle elevation) but is not dependent on cruciate ligament integrity.  Establishment of the quadriceps active position at the quadriceps neutral angle may be used to separate anterior and posterior joint laxity.

Measurement Method

An Arthrometer® is used to measure the anterior/posterior laxity at the quadriceps neutral angle.

1. Apply the Arthrometer® to the normal knee.  An assistant sits beside the patient and supports the leg to obtain the desired relaxation. (The PCL Pro is to be used in place of assistant)

2.  Determine the Quadriceps Neutral Angle, the flexion angle at which there is no tibial anteror/posterior translation with contraction of the quadriceps (mean = 70°, range 55° to 80°).

3. Passive anterior and posterior laxity is measured with a push pull force.

4. Place the injured knee at the Quadriceps Neutral Angle (determined on the normal knee)

5. Measure passive anterior and posterior laxity from the resting position (sag position).

6. Measure the tibial displacement on the Quadriceps Active Test. This equals the distance between the resting position and the quadriceps active position.

7. Calculate the anterior and posterior laxity using the Quadriceps Active Position as the neutral position.  This is done by adding the quadriceps active displacement to the measured posterior laxity and subtracting the quadriceps active displacement from the measured anterior laxity.  The resulting numbers are termed the corrected anterior and posterior laxity measurements.

Determination of Pathologic Laxity

The injured knee laxity should be compared to the patient's normal knee laxity.  A difference of greater then 2 millimeters usually indicates a pathologic condition. Measurements performed at the Quadriceps Neutral Angle from the knee resting position.  Measurements are corrected using the Quadriceps Active Position as the neutral position.
 
 
 

Corrected Side to Side Difference
 
 

Summary

1. The quadriceps active test at 90 degrees of flexion will result in anterior tibial translation of a knee with posterior cruciate ligament disruption.

2. In a normal knee there is a knee flexion position where the quadriceps tendon is normal to the joint surface and contraction of the quadriceps does not result in tibia translation (quadriceps neutral angle.)

3. Measurements of anterior/posterior laxity in the injured knee of a patient may be compared to the normal knee resting position by using the neutral position the quadriceps active position at the quadriceps neutral angle.