Strain Gauge Analysis of Knee Ligaments J. C. KENNEDY, M.D., F.R.C.S. (C),* R. J. HAWKINS, M.D., F.R.C.S. (C)** AND R. B. WILLIS,M.D., F.R.C.S. (C)?
mercury strain gauge was found small enough to fit into the knee joint, sensitive enough to produce the required data and pliable enough not to distort the action of the ligaments. T h e 2 cm gauges were fabricated by filling 30 gauge (0.25 mm. ID) latex rubber tubing with mercury and sealing the bore on each end with silver-tipped 30 gauge wires (Fig. 1 ) . The strain gauges were calibrated by measuring electrical resistance as a function of extension and frequent checking with model 270 plethysmograph from the Park's Electronics Laboratory in Beaverton, Oregon. The strain gauges were obtained from the same laboratory. When sutured into position, the change in length of the ligament is accurately reflected in the mercury column. This resultant change was transcribed onto a model 79 polygraph from the Grass Medical Instruments Company in Quincy. Massachusetts, U. S. The recorder was consistently calibrated to have 5 mm of strain gauge extension equal 100 squares on the graph paper, allowing comparisons among different ligaments and different knees. The force required with the gauges calibrated as above to stretch the gauge 5 mm was determined by an Instron Tension Analyzer Model TT-C. The 3 most important ligamentous stabilizers of' the knee were selected for study: the anterior cruciate, the posterior cruciate and the tibia1 collateral ligaments. To provide a workable baseline on the graph, the strain gauges were sutured in a slightly stretched condition to the mid substance fibres of each ligament. The most lax position in the flexion-extension range as determined by trial and error was selected as the starting point. This point was then taken as the zero point although did not actually represent zero tension. To facilitate the application of gradual controlled forces, the experiments
In the evaluation and treatment of knee ligament injuries and f o r comprehensive understanding of knee ligament function, it is important t o know the relative strains or deformations of different ligaments as a function of joint position. To date, the endless references in the literature regarding knee joint mechanics have been largely qualitative. These are reviewed by Brantigan and Voshell.1 T h e few quantitative analyses have been much t o o limited in their scope t o provide a n accurate appreciation of knee ligament strain.2. 6-8 The impossible task of documenting ligamentous extensibility by direct visualization has led t o many conflicting statements.' T h e a i m of this experiment is t o quantitatively study the relative strains of the major knee ligaments as the knee is subjected t o the various forces approaching the clinical situation.
MATERIALS AND METHODS To clarify this important problem and to quantitatively study knee ligament strain, a *Suite 312, 111 Waterloo Street, London. Ontario Canada. Chairman, Division of Orthopedic Surgery, Department of Surgery, University of Western Ontario, London, Ontario Canada. **450 Central Avenue, London, Ontario Canada. Clinical Assistant Professor, Division of Orthopedic Surgery, Department of Surgery, University of Western Ontario, London, Ontario, Canada. tClinical Fellow, The Hospital for Sick Children, Toronto, Ontario Canada. Received: March 16, 1977. 225
226
Clinical Orthopaedicr and Related Research
Kennedy, Hawkins and Willis
used for the tibial collateral ligament and this wound was likewise not closed. The gauges were attached to the midsubstance of these ligaments (Fig. 1 ) .
RESULTS
FIG.1. Mercury strain gauge sutured in position on midportion of tibial collateral ligament. were carried out on our motorized cadaver stress machine.4 The rate loading system of this machine is extremely slow approaching 10 cm per minute. Multiple tests were then conducted on 5 fresh amputation specimens subjecting Lhese knees to various stresses. The knees were subjected to flexion-extension, rotation, varusvalgus and anteroposterior forces. The height of deflection is expressed in terms of the functional position of the tibia relative to the femur. The anterior cruciate ligament was approached through a long median para-patellar incision, the patella was dislocated and the gauge positioned on the ligament. The wound was closed leaving room for the wires to exit. The posterior cruciate could only be approached from behind, thus disrupting the oblique popliteal ligament and joint capsule. This wound wasn't closed. A direct medial approach was
Strain is the resultant deformation of a structure when subjected to an external load. This deformation is reflected on the polygraph in a quantitative manner. The forces required with the gauges calibrated to have 5 mm of strain gauge extension equal 100 squares on the graph paper was determined by an Instron Tension Analyzer Model TTC. For the purposes of this experiment, this was a linear relationship. Although the results in this experiment have been quantitative, the emphasis is qualitative; that is, in what position does the ligament become taut and roughly by how much. This is expressed in terms of the functional position of the leg. The accompanying graphs demonstrate the relative strains produced by different movements. TIBIAL COLLATERAL LIGAMENT It was determined that the most lax position for the tibial collateral ligament is full flexion. This ligament is stretched as the knee is extended and is maximally elongated
FIG.2. Polygraph recording of tibial collateral ligament (med. coll. ) showing tautness in full extension. Note difference between anterior and posterior fibers.
Number 129 November-December, 1177
Analysis of Knee Ligaments
227
FIG.3. Polygraph recording of anterior cruciate ligament (A.C.1 showing increased tension with forces of hyperextension and hyperflexion and minimal tension at 30 degrees flexion. Note increase in tension with anterior drawer.
in a hyperextended position. This fact is particularly applicable to the posterior and midsubstance fibers but minimally applicable to the anterior fibers. The maximum polygraph deflection is achieved from 0 to 30" of flexion (Fig. 2). At 30" of flexion valgus and external rotation extend the strain gauge whereas varus and internal rotation relax the strain gauge. Of these 2 movements, external rotation has the most significant effect. ANTERIOR CRUCIATE LIGAMENT It was again determined that the most lax position of the anterior cruciate ligament was 35" of flexion. Using this as a starting point, flexion and extension both produced dramatic elongation of the anterior cruciate ligament (Fig. 3 ) . Within the full range of motion, external rotation relaxed the anterior cruciate and this was most marked at approximately 30" of flexion. Similarly, at 30" of flexion, internal rotation moderately tightened the anterior cruciate ligament (Fig. 4 ) . Anterior displacement at 90" tightens while posterior displacement at 90" relaxes the anterior cruciate ligament. In the flexionextension range, the anterior cruciate is already taut at 90" of flexion. At 30" of flexion, valgus mildly relaxes and varus mildly stretches the anterior cruciate ligament.
POSTERIOR CRUCIATE LIGAMENT This ligament is most lax at 35" of flexion. With all movements, this ligament demonstrated the least strain. Flexion and extension forces, however, did dramatically stretch the posterior cruciate ligament. At 120" of flexion where the posterior cruciate is already mildly taut, external and internal rotation both slightly stretch the posterior cruciate ligament. At 30" of flexion, external rotation very minimally stretches this ligament while internal rotation minimally relaxes the ligament. It must be emphasized that the deflection on the graph was very minimal. At 90" of flexion, posterior displacement increased the tension in the ligament whereas anterior displacement had minimal effect. Likewise, varus and valgus forces on the knee at 30" had minimal effect. DISCUSSION Knowledge of the relative tension of a ligament as a function of joint position allows a better understanding of the function and vulnerability of each ligament. These principles can be applied to better understanding of mechanism of injury, diagnosis and treatment. The mechanism of injury of the tibia1 collateral ligament is a result of external rotation and a valgus force applied to the
228
Kennedy, Hswkins and
Clinical Orthopaadicr and Ralatad Raraarch
Willis
a
lpcl
wLr]l i
FIG.4. Polygraph recordings showing increase in tension of anterior cruciate ligament (A.C.) with internal rotation and varus forces and posterior cruciate ( P.C. ) with external and valgus
;:-:;n
WMOIJ
slightly flexed knee.4 Strain gauge analysis has demonstrated that external rotation and valgus put the ligament in a vulnerable position by putting it on the stretch. Likewise, internal rotation and varus relax the ligament. Similarly, if an isolated tear of the anterior cruciate ligament occurs, then one mechanism is an internal rotation and varus force. (One must be aware that with a diagnosis of anterior cruciate tear, microscopic failure may occur in other supporting structures) .5 Hyperextension and hyperflexion also lengthen the anterior cruciate considerably. A hyperextension force produces an isolated tear of the anterior cruciate. The posterior cruciate is also stretched with extension but tension studies show the posterior cruciate is markedly stronger than the anterior cruciate and therefore, protected from injury.j All of these forces mentioned in the mechanism of injury place the ligament in a stretched condition and thus, if any excessive forces are applied, then rupture might ensue. The anterior cruciate is frequently involved in knee ligament injuries associated with other structures. The cadaver stress machine and strain gauge analysis help document that abduction and external rotation first ruptures the capsular and tibial collateral ligaments followed then by the an-
terior c r ~ c i a t eThis . ~ was borne out by selectively cutting the capsular ligament and tibial collateral ligament and then externally rotating the knee to show that the anterior cruciate first became relaxed within the normal range of motion and then taut as the stability provided by the medial structures was exceeded. It was at this stage that the anterior cruciate ligament ruptured. In our experiments, the posterior cruciate was the most difficult to instrument and was subjected to the least extension with forces on the knee, particularly rotation, varusvalgus and anteroposterior forces. Hughston has called this ligament the primary stabilizer of the knee3 as our strain gauge analysis has confirmed. At 90", the posterior cruciate ligament is already taut and superimposed internal or external rotation further slightly stretched the posterior cruciate ligament. In addition, tension studies on the posterior cruciate revealed that it is considerably stronger than the anterior cruciate and this combined with its size and its position near the central axis of the knee would protect it from rupture.5 According to this experiment, it is unlikely any forces would disrupt the posterior cruciate without first tearing another major structure. The exception to this is with an isolated posterior displacement of the tibia on the femur.
Number 129 November-December, 1917
SUMMARY Mercury strain gauges were sutured onto the tibial collateral anterior and posterior cruciate ligaments to quantitatively determine the relative strain or deformation of each of these ligaments as a function of joint position. The results were obtained on 5 amputation specimens by subjecting them to flexion, extension, rotation, valgus-varus and anteroposterior forces. The tibial collateral ligament is most lax in full flexion and stretches with extension, valgus and external rotation. The cruciate ligaments are most lax at 35' flexion and stretch with both flexion and extension. Internal rotation and varus stretch the anterior cruciate ligament. These principles allow us a better understanding of injury patterns. The most advantageous position for immobilization following acute injuries or reconstructions is better understood knowing that minimal tension on ligamentous fibers occurs as follows: Anterior cruciate, 35"; Posterior cruciate, 35 O ; Tibia1 collateral ligament, 45-90' (or as much flexion as the patient will tolerate).
Analysis of Knee Ligaments
229
REFERENCES 1. Brantigdn, 0. C. and Voshell, A. F.: The
2. 3.
4.
5.
6. 7.
8.
mechanics of the lieaments and menisci of the knee joint. J. Bone and Surg. 23:44, 1941. Edwards, R. G.. Lafferty, J. F. and Lounge, K. 0.: Ligament strain in the human knee joint, J. Basic Eng. 131, 1970. Hughston, J. C.: The posterior cruciate ligament in knee joint stability, J. Bone Joint Surg. 51-A: 1045, 1969. Kennedy, J. C. and Fowler, P. J.: Medial and anterior instability of the knee. An anatomical and clinical study using stress machines. J. Bone Joint Surg. 53-A:I257, 1971. Kennedy, J. C., Hawkins, R. J., Willis, B. and Danylchuck, K.: Tension studies of human knee ligaments. Yield point, ultimate failure and disruption of the cruciate and tibial collateral ligaments, J. Bone Joint Surg. 58-A: 350, 1976. Smith, J. W.: The elastic properties of the anterior cruciate of the rabbit, J. Anat. 369, 1954. Viidik, A.: A rheological model for uncalcified parallel fibered collagenous tissue, J. Biornech. 1:3, 1968. White, A. A. and Raphael, I. G.: The effect of quadriceps loads and knee position on strain measurements of the tibial collateral ligament, Acta. Orthop. Scand. 43: 176, 1972.
mercury strain gauge was found small enough to fit into the knee joint, sensitive enough to produce the required data and pliable enough not to distort the action of the ligaments. T h e 2 cm gauges were fabricated by filling 30 gauge (0.25 mm. ID) latex rubber tubing with mercury and sealing the bore on each end with silver-tipped 30 gauge wires (Fig. 1 ) . The strain gauges were calibrated by measuring electrical resistance as a function of extension and frequent checking with model 270 plethysmograph from the Park's Electronics Laboratory in Beaverton, Oregon. The strain gauges were obtained from the same laboratory. When sutured into position, the change in length of the ligament is accurately reflected in the mercury column. This resultant change was transcribed onto a model 79 polygraph from the Grass Medical Instruments Company in Quincy. Massachusetts, U. S. The recorder was consistently calibrated to have 5 mm of strain gauge extension equal 100 squares on the graph paper, allowing comparisons among different ligaments and different knees. The force required with the gauges calibrated as above to stretch the gauge 5 mm was determined by an Instron Tension Analyzer Model TT-C. The 3 most important ligamentous stabilizers of' the knee were selected for study: the anterior cruciate, the posterior cruciate and the tibia1 collateral ligaments. To provide a workable baseline on the graph, the strain gauges were sutured in a slightly stretched condition to the mid substance fibres of each ligament. The most lax position in the flexion-extension range as determined by trial and error was selected as the starting point. This point was then taken as the zero point although did not actually represent zero tension. To facilitate the application of gradual controlled forces, the experiments
In the evaluation and treatment of knee ligament injuries and f o r comprehensive understanding of knee ligament function, it is important t o know the relative strains or deformations of different ligaments as a function of joint position. To date, the endless references in the literature regarding knee joint mechanics have been largely qualitative. These are reviewed by Brantigan and Voshell.1 T h e few quantitative analyses have been much t o o limited in their scope t o provide a n accurate appreciation of knee ligament strain.2. 6-8 The impossible task of documenting ligamentous extensibility by direct visualization has led t o many conflicting statements.' T h e a i m of this experiment is t o quantitatively study the relative strains of the major knee ligaments as the knee is subjected t o the various forces approaching the clinical situation.
MATERIALS AND METHODS To clarify this important problem and to quantitatively study knee ligament strain, a *Suite 312, 111 Waterloo Street, London. Ontario Canada. Chairman, Division of Orthopedic Surgery, Department of Surgery, University of Western Ontario, London, Ontario Canada. **450 Central Avenue, London, Ontario Canada. Clinical Assistant Professor, Division of Orthopedic Surgery, Department of Surgery, University of Western Ontario, London, Ontario, Canada. tClinical Fellow, The Hospital for Sick Children, Toronto, Ontario Canada. Received: March 16, 1977. 225
226
Clinical Orthopaedicr and Related Research
Kennedy, Hawkins and Willis
used for the tibial collateral ligament and this wound was likewise not closed. The gauges were attached to the midsubstance of these ligaments (Fig. 1 ) .
RESULTS
FIG.1. Mercury strain gauge sutured in position on midportion of tibial collateral ligament. were carried out on our motorized cadaver stress machine.4 The rate loading system of this machine is extremely slow approaching 10 cm per minute. Multiple tests were then conducted on 5 fresh amputation specimens subjecting Lhese knees to various stresses. The knees were subjected to flexion-extension, rotation, varusvalgus and anteroposterior forces. The height of deflection is expressed in terms of the functional position of the tibia relative to the femur. The anterior cruciate ligament was approached through a long median para-patellar incision, the patella was dislocated and the gauge positioned on the ligament. The wound was closed leaving room for the wires to exit. The posterior cruciate could only be approached from behind, thus disrupting the oblique popliteal ligament and joint capsule. This wound wasn't closed. A direct medial approach was
Strain is the resultant deformation of a structure when subjected to an external load. This deformation is reflected on the polygraph in a quantitative manner. The forces required with the gauges calibrated to have 5 mm of strain gauge extension equal 100 squares on the graph paper was determined by an Instron Tension Analyzer Model TTC. For the purposes of this experiment, this was a linear relationship. Although the results in this experiment have been quantitative, the emphasis is qualitative; that is, in what position does the ligament become taut and roughly by how much. This is expressed in terms of the functional position of the leg. The accompanying graphs demonstrate the relative strains produced by different movements. TIBIAL COLLATERAL LIGAMENT It was determined that the most lax position for the tibial collateral ligament is full flexion. This ligament is stretched as the knee is extended and is maximally elongated
FIG.2. Polygraph recording of tibial collateral ligament (med. coll. ) showing tautness in full extension. Note difference between anterior and posterior fibers.
Number 129 November-December, 1177
Analysis of Knee Ligaments
227
FIG.3. Polygraph recording of anterior cruciate ligament (A.C.1 showing increased tension with forces of hyperextension and hyperflexion and minimal tension at 30 degrees flexion. Note increase in tension with anterior drawer.
in a hyperextended position. This fact is particularly applicable to the posterior and midsubstance fibers but minimally applicable to the anterior fibers. The maximum polygraph deflection is achieved from 0 to 30" of flexion (Fig. 2). At 30" of flexion valgus and external rotation extend the strain gauge whereas varus and internal rotation relax the strain gauge. Of these 2 movements, external rotation has the most significant effect. ANTERIOR CRUCIATE LIGAMENT It was again determined that the most lax position of the anterior cruciate ligament was 35" of flexion. Using this as a starting point, flexion and extension both produced dramatic elongation of the anterior cruciate ligament (Fig. 3 ) . Within the full range of motion, external rotation relaxed the anterior cruciate and this was most marked at approximately 30" of flexion. Similarly, at 30" of flexion, internal rotation moderately tightened the anterior cruciate ligament (Fig. 4 ) . Anterior displacement at 90" tightens while posterior displacement at 90" relaxes the anterior cruciate ligament. In the flexionextension range, the anterior cruciate is already taut at 90" of flexion. At 30" of flexion, valgus mildly relaxes and varus mildly stretches the anterior cruciate ligament.
POSTERIOR CRUCIATE LIGAMENT This ligament is most lax at 35" of flexion. With all movements, this ligament demonstrated the least strain. Flexion and extension forces, however, did dramatically stretch the posterior cruciate ligament. At 120" of flexion where the posterior cruciate is already mildly taut, external and internal rotation both slightly stretch the posterior cruciate ligament. At 30" of flexion, external rotation very minimally stretches this ligament while internal rotation minimally relaxes the ligament. It must be emphasized that the deflection on the graph was very minimal. At 90" of flexion, posterior displacement increased the tension in the ligament whereas anterior displacement had minimal effect. Likewise, varus and valgus forces on the knee at 30" had minimal effect. DISCUSSION Knowledge of the relative tension of a ligament as a function of joint position allows a better understanding of the function and vulnerability of each ligament. These principles can be applied to better understanding of mechanism of injury, diagnosis and treatment. The mechanism of injury of the tibia1 collateral ligament is a result of external rotation and a valgus force applied to the
228
Kennedy, Hswkins and
Clinical Orthopaadicr and Ralatad Raraarch
Willis
a
lpcl
wLr]l i
FIG.4. Polygraph recordings showing increase in tension of anterior cruciate ligament (A.C.) with internal rotation and varus forces and posterior cruciate ( P.C. ) with external and valgus
;:-:;n
WMOIJ
slightly flexed knee.4 Strain gauge analysis has demonstrated that external rotation and valgus put the ligament in a vulnerable position by putting it on the stretch. Likewise, internal rotation and varus relax the ligament. Similarly, if an isolated tear of the anterior cruciate ligament occurs, then one mechanism is an internal rotation and varus force. (One must be aware that with a diagnosis of anterior cruciate tear, microscopic failure may occur in other supporting structures) .5 Hyperextension and hyperflexion also lengthen the anterior cruciate considerably. A hyperextension force produces an isolated tear of the anterior cruciate. The posterior cruciate is also stretched with extension but tension studies show the posterior cruciate is markedly stronger than the anterior cruciate and therefore, protected from injury.j All of these forces mentioned in the mechanism of injury place the ligament in a stretched condition and thus, if any excessive forces are applied, then rupture might ensue. The anterior cruciate is frequently involved in knee ligament injuries associated with other structures. The cadaver stress machine and strain gauge analysis help document that abduction and external rotation first ruptures the capsular and tibial collateral ligaments followed then by the an-
terior c r ~ c i a t eThis . ~ was borne out by selectively cutting the capsular ligament and tibial collateral ligament and then externally rotating the knee to show that the anterior cruciate first became relaxed within the normal range of motion and then taut as the stability provided by the medial structures was exceeded. It was at this stage that the anterior cruciate ligament ruptured. In our experiments, the posterior cruciate was the most difficult to instrument and was subjected to the least extension with forces on the knee, particularly rotation, varusvalgus and anteroposterior forces. Hughston has called this ligament the primary stabilizer of the knee3 as our strain gauge analysis has confirmed. At 90", the posterior cruciate ligament is already taut and superimposed internal or external rotation further slightly stretched the posterior cruciate ligament. In addition, tension studies on the posterior cruciate revealed that it is considerably stronger than the anterior cruciate and this combined with its size and its position near the central axis of the knee would protect it from rupture.5 According to this experiment, it is unlikely any forces would disrupt the posterior cruciate without first tearing another major structure. The exception to this is with an isolated posterior displacement of the tibia on the femur.
Number 129 November-December, 1917
SUMMARY Mercury strain gauges were sutured onto the tibial collateral anterior and posterior cruciate ligaments to quantitatively determine the relative strain or deformation of each of these ligaments as a function of joint position. The results were obtained on 5 amputation specimens by subjecting them to flexion, extension, rotation, valgus-varus and anteroposterior forces. The tibial collateral ligament is most lax in full flexion and stretches with extension, valgus and external rotation. The cruciate ligaments are most lax at 35' flexion and stretch with both flexion and extension. Internal rotation and varus stretch the anterior cruciate ligament. These principles allow us a better understanding of injury patterns. The most advantageous position for immobilization following acute injuries or reconstructions is better understood knowing that minimal tension on ligamentous fibers occurs as follows: Anterior cruciate, 35"; Posterior cruciate, 35 O ; Tibia1 collateral ligament, 45-90' (or as much flexion as the patient will tolerate).
Analysis of Knee Ligaments
229
REFERENCES 1. Brantigdn, 0. C. and Voshell, A. F.: The
2. 3.
4.
5.
6. 7.
8.
mechanics of the lieaments and menisci of the knee joint. J. Bone and Surg. 23:44, 1941. Edwards, R. G.. Lafferty, J. F. and Lounge, K. 0.: Ligament strain in the human knee joint, J. Basic Eng. 131, 1970. Hughston, J. C.: The posterior cruciate ligament in knee joint stability, J. Bone Joint Surg. 51-A: 1045, 1969. Kennedy, J. C. and Fowler, P. J.: Medial and anterior instability of the knee. An anatomical and clinical study using stress machines. J. Bone Joint Surg. 53-A:I257, 1971. Kennedy, J. C., Hawkins, R. J., Willis, B. and Danylchuck, K.: Tension studies of human knee ligaments. Yield point, ultimate failure and disruption of the cruciate and tibial collateral ligaments, J. Bone Joint Surg. 58-A: 350, 1976. Smith, J. W.: The elastic properties of the anterior cruciate of the rabbit, J. Anat. 369, 1954. Viidik, A.: A rheological model for uncalcified parallel fibered collagenous tissue, J. Biornech. 1:3, 1968. White, A. A. and Raphael, I. G.: The effect of quadriceps loads and knee position on strain measurements of the tibial collateral ligament, Acta. Orthop. Scand. 43: 176, 1972.
