Journal of Orthopuedic Research 13:442-449 The Journal of Bone and Joint Surgery, Inc 0 1995 Orthopaedic Research Society

Healing of the Medial Collateral Ligament After a Combined Medial Collateral and Anterior Cruciate Ligament

Injury and Reconstruction of the Anterior Cruciate Ligament: Comparison of Repair and Nonrepair

of Medial Collateral Ligament Tears in Rabbits

Kazunori Ohno, Amy S. Pomaybo, Christopher C. Schmidt, Rebecca E. Levine, Karen J. Ohland, and Savio L-Y. Woo

Musculoskeletal Research Center, Departrrient of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S. A.

Summary: The optimal treatment for a combined injury of the medial collateral and anterior cruciate liga- ments is controversial. and the question remains as to whether repair of the medial collateral ligament and reconstruction of the anterior cruciate ligament improves healing of the medial collateral ligament. We compared reconstruction of the anterior cruciate ligament with and without repair of the medial collateral ligament in a rabbit model of a combined injury of these two ligaments. The anterior-posterior translation and varus-valgus rotation of the knee, the structural properties of the femur-medial collateral ligament-tibia complex, and the mechanical properties of the midsubstance of the medial collateral ligament were evaluated immediately after surgery and at 6 and 12 weeks postoperatively. Repair of the medial collateral ligament led to significantly less varus-valgus rotation of the knee than did no repair, but the anterior-posterior translation of the knees in the repair and nonrepair groups were not significantly different at any study time. At 12 weeks, the cross-sectional area and ultimate load in the repair group were 60 and 53% greater, respectively, than in the nonrepair group. Among 12 specimens that were repaired (six specimens at 6 weeks and six specimens at 12 weeks), failure occurred within the midsubstance in four (two at each time period); in all of the specimens that were not repaired, failure occurred at the tibial insertion site. There was no significant difference between the modulus of the midsubstance in the repaired and the nonrepaired medial collateral ligaments. Thus, the improved structural properties of the femur-medial collateral ligament-tibia complexes that were repaired resulted from an increase in cross-sectional area of the repaired medial collateral ligament and healing of the tibial insertion site. Postoperative healing time had little effect on the tensile properties. In this rabbit model, repair of the medial collateral ligament with reconstruction of the anterior cruciate ligament may lead to better healing of the medial collateral ligament in the early phase than does reconstruc- tion of the anterior cruciate ligament alone.

The medial collateral ligament is one of the most frequently injured ligaments of the knee (19). There- fore, many studies have been performed to evalu- a te treatment regimens for injuries t o this ligament. For many years, most complete isolated tears of the medial collateral ligament were treated surgically (8,18,20,27), but recent clinical results have supported nonoperative management (3,13,22.32). In addition, experimental studies have demonstrated that a n iso-

Received Februdry 14,1994, accepted July 1 I , 1994. Address correspondence and reprint requests to S L-Y Woo

at Department of Orthopaedic Surgery. University of Pittsburgh, 101 1 Liliane S. Kaufmann Building. 3471 Fifth Avenue. Pittsburgh. PA 15213. U S.A.

lated rupture of the medial collateral ligament heals well without surgical intervention; the increase in varus-valgus laxity of the knee is minimal, and there is little evidence of subsequent damage to the knee joint (14,28). In more severe injuries, such as a com- bined injury of the medial collateral and anterior cruciate ligaments or an O’Donoghue triad injury (1,9,20), the optimal treatment for tears of the medial collateral ligament has yet t o be established. Several authors have endorsed repair of the medial collateral ligament with reconstruction of the anterior cruciate ligament for a combined ligament injury (15,21,26.32). Others, however, have recommended reconstruction of the anterior cruciate ligament alone; their conten- tion is that restoration of the function of the anterior

442

44.7 M C L HEALING I N A COMBINED MCL A N D ACL I N J U R Y

Dahners (16) found that exercise increased the ul- timate load of the healed femur-medial collateral ligament-tibia complex but also increased laxity of the medial collateral ligament.

Thus, whether surgical repair of tears of the medial collateral ligament in a combined ligament injury im- proves the healing of the medial collateral ligament remains an open question. The purpose of this study was to compare reconstruction of the anterior cruciate ligament with and without repair of the medial collat- eral ligament after a combined ligament injury in rab- bits by examining the gross appearance of the joint. anterior-posterior translation and varus-valgus rota- tion of the knee, cross-sectional area of the ligament, structural properties and mode of failure of the healed femur-medial collateral ligament-tibia complex, and mechanical properties of the healed medial collateral ligament.

a

MATERIALS AND METHODS Forty-two skeletally mature New Zealand White rabbits. weigh-

ing 4.0 -t 0 S kg (mean f SD), were used. Each animal was given an intramuscular injection of ketamine and acepromazine and of cephalothin sodium prior to surgery. All operation$ were per- formed under sterile conditions and with the animal under general aneqthesia

In each animal. a longitudinal anterolateral skin incision was made to expose the knee joint of the left hindlimb The medial collateral ligament was undermined, and a 2 5 mm diameter rod

FIG. 1. Procedure for reconstruction of the anterior cruciate liga- ment using a digital flexor tendon allograft. (a) The graft is placed intra-articularly through a tibia1 drill-hole and over the top of the lateral femoral condyle (b and c). and then it is fixed by a staple and sutures (d).

cruciate ligament effectively converts the combined injury to an isolated injury of the medial collateral ligament (4,23,24).

Experimental studies of healing of the medial col- lateral ligament in a model of a combined ligament injury have shown that the quality of the healed medial collateral ligament after nonoperative treatment of both ligaments is worse than that of an untreated medial collateral ligament after an isolated injury in which the anterior cruciate ligament is left intact (6,10,30). Other studies have examined the effects of immobilization, surgical repair, and exercise on healing of the medial collateral ligament in the combined lig- ament injury model. In the rabbit. Bray et al. (7) found that immobilization of the knee prevented increased laxity of the medial collateral ligament, but the stiffness and ultimate load of the healed femur-medial collateral ligament-tibia complex did not return to normal. Using a rat model, Hart and Dahners (12) showed that re- pair of tears of the medial collateral ligament did not significantly affect laxity of the medial collateral lig- ament or ultimate load of the healed femur-medial colIateral ligament-tibia complex, and Lechner and

was placed beneath it. Two small incisions were made at the ante- rior and posterior edges of the midportion of the medial collateral ligament. and the ligament then was ruptured by pulling medially

--h \

FIG. 2. Procedure for repair of the ruptured medial collateral lig- ament using a modified Kessler technique (25) with a 3-0 braided nylon suture (a and c), reinforced by two simple 5-0 braided nylon sutures (h and c).

J Orthop Xes. Vol. 13, No. 3, 1995

K . OHNO E T AL.

cephalothin sodium (0.5 g) were administered twice a day for 3 days postoperatively or until no longer needed. Six animals from the repair group and six from the nonrepair group were killed immediately after surgery and at 6 and 12 weeks postoperatively. Each hindlimb was disarticulated at the hip joint, wrapped in saline-soaked gauze, placed in double plastic bags, and stored at -20°C. Prior to biomechanical testing, the specimens were thawed overnight at 4°C. The surrounding tissues of the knee joint were removed, and care was taken not to injure the joint capsule or ligaments. The specimens were kept moist with 0.9% saline solu- tion during dissection and biomechanical testing.

Anterior-posterior translation of the intact knee was measured using a custom-built device that permits 4 degrees of freedom: anterior-posterior, medial-lateral, and proximal-distal translations and axial tibial rotation (9). The specimen was mounted in this apparatus with the knee in 90" of flexion. A cyclic load of *lo N was applied perpendicular to the tibia while a linear variable differential transformer measured the corresponding anterior- posterior translation.

Varus-valgus rotation of the knee was measured using an- other custom-built device that allows varus-valgus rotation as well as medial-lateral and proximal-distal translations of the knee (9,28). With the knee flexed at 90", two pins were placed sagittally through the femur and tibia and were secured to the device. A bending moment of 20.1 Nm was applied perpendic- ular to the medial collateral ligament while two rotary variable differential transformers measured the corresponding varus- valgus rotation.

After the varus-valgus rotation had been measured. all struc- tures except the medial collateral ligament were resected, leaving the femur-medial collateral ligament-tibia complex. The cross- sectional area of the medial collateral ligament was measured at three locations (the joint line and 3 mm proximal to and 3 mm distal to the joint line) using a laser micrometer system (31). To provide a clear path for the laser beam, 3 mm each of the femoral and tibial articular surfaces were removed using a saw. Care was taken to avoid damage to the ligament at the tibial and femoral insertion sites. The medial collateral ligament was placed perpen- dicular to the collimated laser beam and rotated through 180" at 3" increments. Its width at each increment was measured using a computer algorithm (31). These data were compiled to reconstruct

- MCL+ACL Deficient I Repair

- r T

- r T

OWeeks 6Weeks 12Weeks FIG. 3. Anterior-posterior (A-P) translation (mean 2 SD) of the knee in the repair and nonrepair groups immediately after surgery (0 weeks) and at 6 and 12 weeks, expressed as a ratio of experi- mental to control (E/C) values. For comparison, the mean value for the ligament-deficient group (MCL+ACL deficient) is shown. MCL = medial collateral ligament and ACL = anterior cruciate ligament.

on the rod. The capsule was left intact. This injury model, estab- lished in our laboratory, produces a consistent injury of the mid- substance with simultaneous damage to the ligament insertions to the bone (1,9,28). A lateral parapatellar arthrotomy then was made. and the anterior cruciate ligament was transected at its tibial insertion. Six rabbits with this combined injury (ligament-deficient group) were killed immediately after surgery and were used to obtain baseline values for anterior-posterior and varus-valgus in- stability of the knee.

In the remaining 36 rabbits, the anterior cruciate ligament was reconstructed using a digital flexor tendon allograft (Fig. 1). The allografts had been obtained aseptically from the hind- limbs of freshly killed rabbits of the same species and age as the study group and were frozen at -70°C until needed. Each graft was allowed to thaw at room temperature and then was trimmed to approximately 4 mm in width and 6 cm in length before use in the reconstruction. A custom-made stainless-steel button (8 mm diameter) was secured to one end of the graft with two 2-0 nylon sutures. A straight needle was attached to the other end of the graft with a 2-0 nylon suture. A 3 mm tunnel was drilled in the tibia from the anteromedial cortex toward the distal insertion site of the anterior cruciate ligament. The needle was used to guide the graft through the tibial tunnel and into the knee joint, where the graft was placed over the top of the lateral femoral condyle. Ten newtons of tension, mea- sured with a fish-weight scale, was applied to the graft ( 5 ) , and this force was maintained while the graft was secured to the lateral femoral cortex with a titanium staple (7 x 10 mm; 3M, St. Paul, MN, U.S.A.) and four 3-0 braided nylon sutures.

In the repair group (18 animals), the ruptured medial collateral ligament was repaired with a 3-0 braided nylon suture using a modified Kessler technique (25) and was reinforced by two simple 5-0 braided nylon sutures (Fig. 2). In the remaining 18 animals (nonrepair group), the ends of the ruptured medial collateral lig- ament were apposed but not repaired.

All wounds were closed routinely. In the right hindlimb of all 42 animals the medial collateral ligament was exposed and under- mined but not ruptured, and a lateral parapatellar arthrotomy was made. These knees served as contralateral controls.

After surgery, all of the animals were allowed unrestricted ac- tivity in cages (area, 4,000 cm2). Butorphanol tartrate (0.4 ml) and

MCL+ACL Deficient 0 Repair

h Nonrepair r i

T

OWeeks 6Weeks 12Weeks FIG. 4. Varus-valgus (V-V) rotation (mean 2 SD) of the knee in the repair and nonrepair groups immediately after surgery (0 weeks) and at 6 and 12 weeks, expressed as a ratio of experimental to control (E/C) values. For comparison, the mean value for the ligament-deficient group (MCL+ACL deficient) is shown. MCL = medial collateral ligament and ACL = anterior cruciate ligament. *Significant difference (p < 0.05).

J Orthop Res, Vol. 13, No. 3, 1W5

MCL HEALING IN A COMBINED MCL A N D ACL INJURY 445

TABLE 1. Structural properties (mean 2 SD) of the femur-medial collateral ligament-tibia complex at 6 and 12 weeks

6 weeks 12 weeks Control (n = 6) Experimental (n = 6) Control (n = 6) Experimental (n = 6)

Repair group Cross-sectional area (mm’) 3.4 t 0.4 10.1 t 2.0 3.9 t 0.5 9.6 2 2.4 Stiffness (Nlmm) 41.5 2 5.8 19.4 2 8.4 44.0 2 11.2 26.0 2 6.5 Ultimate load (N) 304 2 65.4 99.2 t 35.5 283 ? 74.0 136 f 39.0” Elongation at failure (mm) 7.4 t 0.5 5.5 t 1.0 7.5 t 0.9 5.7 t 0.6 Energy absorbed (N-mm) 1.031 ? 281 281 -C 113 1,072 t 402 345 2 103

Nonrepair group Cross-sectional area (mmz) 3.4 t 0.8 7.8 2 2.6 Stiffness (N/mm) 44.5 t 10.3 19.6 2 6.2 Ultimate load (N) 308 f 56.6 82.9 t 43.7 Elongation at failure (mm) 7.3 2 1.1 4.7 2 0.8 Energy absorbed (N-mm) 1,082 -C 315 184 2 126

3.6 2 1.3 43.1 t 7.7 261 2 35.0 6.8 ? 0.7 838 f 132

6.0 2 1.9 20.8 f 3.9 88.9 t 32.3 4.9 t 1.1 209 2 55

Significantly different (p < 0.05) from the value for the nonrepair group at 12 weeks.

the cross-sectional shape of the medial collateral ligament. Using Simpson’s rule (2), the cross-sectional area was calculated to within 0.1 mmz of accuracy.

For tensile testing, the femur-medial collateral ligament-tibia complex then was mounted in custom-designed clamps and im- mersed in a 37°C saline bath, attached to a materials testing ma- chine (model 4502; Instron, Canton, MA, U.S.A.). After a preload of 0.5 N was applied. each femur-medial collateral ligament-tibia complex was preconditioned by cycling between 0 and 1 mm of elongation for 10 cycles and then was loaded to failure at a rate of 10 m m h i n (29). A load-elongation curve was recorded for each specimen, and the mode of failure was noted. The structural prop- erties of the complex, represented by stiffness. ultimate load, elon- gation at failure, and energy absorbed to failure. were calculated from the load-elongation curves. Stiffness was determined be- tween 1 and 3 mm of elongation.

To measure strain in the midsubstance of the medial collat- eral ligament, three black circular markers (2 mm diameter) were placed on the midsubstance, approximately 3 mm apart along the long axis of the ligament. A motion analysis system (Motion Analysis Corporation. Santa Rosa, CA, U.S.A.) was used for continual, simultaneous tracking of the centroid positions of all three markers throughout the tensile test (17). Strain was de- termined by normalizing the change in distance (AULo) between the two markers closest to the failure site. To calculate the stress, the load was divided by the previously measured cross-sectional area closest to the failure site. Thus. a stress-strain curve for the midsubstance of the medial collateral ligament was obtained to represent the mechanical properties. From this curve, modulus (between 3 and 5% strain), tensile strength, and strain at failure were determined.

Two-way analysis of variance was used to assess the effects of postoperative healing time and repair of the medial collat- eral ligament on anterior-posterior translation and varus-valgus rotation of the knee, cross-sectional area of the ligament, struc- tural properties of the femur-medial collateral ligament-tibia complex, and mechanical properties of the medial collateral lig- ament substance. Paired t tests were used to compare the differ- ences in these properties between the experimental and control specimens. The significance level for the two-way analysis of variance and paired t test was set at p < 0.05, and the Bon- ferroni method was used to calculate the significance level for subsequent unpaired f tests.

RESULTS The medial collateral ligaments from all control

knees appeared shiny, white, and opaque, with well

defined edges. At 6 weeks, a translucent material bridged the gap in the ruptured medial collateral lig- aments from the repair and nonrepair groups; the bor- ders between the original and new ligament tissue

6 W e e k s

4001 -m- Control T -0- Repair -L Nonrepair T ,

300{

-1 0 2 4 6 8

0

Elongation (mm)

12 W e e k s

300 400)

Elongation(mm) FIG. 5. Load-elongation curves (mean 5 SD) for the femur-medial collateral ligament-tibia complexes at 6 and 12 weeks.

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446 K . OHNO ET AL.

.

60- h

2 E

TABLE 2. Mechanical properties (mean 2 SD) of the midsubstance of the medial collateral ligument

-Control -0- Repair +Nonrepair -

T

, ,

6 weeks 12 weeks

Control (n = 6) Experimental (n = 6) Control (n = 6) Experimental (n = 6)

Repair group Modulus (MPa) 843 2 533 129 % 74

Strain at failure (Yo) 10.8 t- 4.2 14.5 -+- 5.5"

Modulus (MPa) 1,044 i 432 94 % 60

Tensile strength (MPa) 88.2 2 11.9 7.7 i 2.8"

Nonrepair group

Tensile strength (MPa) 94.2 ? 22.5 -

Strain at failure (%) 9.0 2 3.9 -

833 2 484 71.5 2 17.1 12.2 t 4.0

168 2 70 16.6 % 1.0 13.0 2 10.9"

813 2 664 84.5 t 30.7* - I I .1 2 3.gh

141 2 51

-

No experimental specimens were available at 6 or 12 weeks for testing of tensile strength and strain at failure because all specimens had

"N = 2 due to avulsion at the tibial insertion site in four specimens. h N = 5 due to avulsion at the femoral insertion site in one specimen.

avulsion at the tibial insertion site.

were barely discernible. This translucent mass ap- peared to be larger in the repaired medial collateral ligaments than in the nonrepaired ones. At 12 weeks, the healing sites remained enlarged and similar in appearance to those examined at 6 weeks. In all of the experimental specimens from both groups at both time periods, the fascia1 layer adhered to the ligament surface, and small osteophytes had formed in the me- dial periphery of the tibiofemoral joint. The anterior cruciate ligament allografts in the two groups had a similar appearance: they were white and less glossy than normal anterior cruciate ligaments at 6 weeks. By 12 weeks, the allografts had become rounder and slightly swollen, but there was no synovial covering or blood vessels visible on their surface; in each 12-week group one graft was torn.

The mean (+-SD) anterior-posterior translation of the knee was 1.3 C 0.2 mm in the control group and 8.1 -C 0.6 mm in the ligament-deficient group. The an terior-posterior translations immediately after sur- gery were 3.5 2 0.9 mm (repair group) and 3.3 +- 0.6 mm (nonrepair group). At 6 weeks, these values were 4.8 % 0.7 and 4.8 -+ 1.1 mm and at 12 weeks, they were 5.0 -+ 1.3 and 5.2 5 0.7 mm. To minimize interanimal variation, the anterior-posterior translations were nor- malized and expressed as a ratio of experimental to control values (Fig. 3). Two-way analysis of variance revealed that time had a significant effect on the an- terior-posterior translation between 0 and 12 weeks (p < 0.05), but repair of the medial collateral ligament had no significant effect (p > 0.05); no significant in- teraction was found between time and repair of the medial collateral ligament (p > 0.05).

Varus-valgus rotation of the knee was 5.7 -t 1.1" in the control group and 22.3 % 5.1" in the ligament- deficient group. The varus-valgus rotations in the re- pair and nonrepair groups were 6.1 2 1.3" and 12.5 5 3.2" immediately after reconstruction of the anterior cruciate ligament, 7.7 f- 2.2" and 11.7 2 2.8" at 6 weeks, and 9.7 f- 2.1" and 14.8 ? 4.3" at 12 weeks. The varus-valgus rotations also were expressed as a ratio

of experimental to control values (Fig. 4). Two-way analysis of variance demonstrated that repair of the medial collateral ligament had a significant effect on the varus-valgus rotation (p < 0.001) but that time had no significant effect (p > 0.05); no significant interac-

801 60

I2 Weeks

T

Strain (%) FIG. 6. Stress-strain curves (mean 5 SD) for the substance of the medial collateral ligaments at 6 and 12 weeks.

3 Orthop Res, Vol. 13, No. 3, 1995

MCL HEALING IN A COMBINED MCL A N D ACL INJURY 44 7

tion was found between repair of the medial collateral ligament and time (p > 0.05).

At each time period, the cross-sectional area of the medial collateral ligament was significantly larger in both experimental groups than in the control group (p < 0.05) (Table 1). At 6 weeks, the values for the repaired and nonrepaired ligaments were 3.0 and 2.3 times greater than the control values and at 12 weeks, they were 2.5 and 1.7 times greater than the control values. At 12 weeks, the areas of the repaired liga- ments were significantly larger than those of the non- repaired ones (p < 0.05). The area was not significantly affected by postoperative time (p > 0.05), nor was there a significant interaction between time and repair of the ligament (p > 0.05).

All of the values for stiffness, ultimate load, elonga- tion at failure, and energy absorbed to failure in the experimental femur-medial collateral ligament-tibia complexes were significantly lower than the corre- sponding values in the control complexes (p < 0.05) (Table 1). At 6 and 12 weeks, the values for ultimate load and energy absorbed to failure were greater in the repair group than in the nonrepair group, but the differences were not significant (Fig. 5) . At 12 weeks, the values for all of the structural properties were greater in the repair group than in the nonrepair group, but only the difference in ultimate load was statistically significant (p < 0.05). Two-way analysis of variance demonstrated no significant effect of time on any of the structural properties (p > 0.05) and no significant interaction between repair of the medial collateral ligament and time (p > 0.05).

At 6 and 12 weeks, failure occurred within the mid- substance of the medial collateral ligament in all of the control complexes except one (at 12 weeks), in which failure was caused by avulsion at the femoral insertion site. In 20 of the 24 experimental complexes, failure occurred by avulsion at the tibial insertion site, which indicates damage to the ligament at this site (1,9,28). In two complexes in the repair groups at both 6 and 12 weeks, failure occurred within the midsubstance.

The modulus of the medial collateral ligament was significantly different in the experimental and control groups at both time periods (p < 0.05) (Table 2 and Fig. 6). The modulus of the ligaments in the repair group was 15% of the control value at 6 weeks and 20% at 12 weeks. Similarly, the modulus of the liga- ments in the nonrepair group was 9% of the control value at 6 weeks and 17% at 12 weeks. There were no significant effects of time or repair on the modulus (p > 0.05), and no interaction between these effects was found (p > 0.05). The effect of time and repair on the tensile strength and strain at failure could not be evaluated statistically because of the insufficient num- bers of experimental specimens in which failure oc-

curred in the midsubstance of the medial collateral ligament (Table 2).

DISCUSSION For a combined injury of the medial collateral

and anterior cruciate ligaments, reconstruction of the anterior cruciate ligament consistently has been recommended (4,15,21,23,24,26,32), but the optimal treatment of the ruptured medial collateral liga- ment has not been established. In this study, we cre- ated a rabbit model of a combined injury of these two ligaments, reconstructed the anterior cruciate ligament, and then examined the effects of repair of the medial collateral ligament. In this model, re- pair of the medial collateral ligament led to less varus-valgus instability of the knee and improved healing of the femur-medial collateral ligament-tibia complex, but it did not have a significant effect on anterior-posterior instability of the knee and the modulus of the healed medial collateral ligament.

Reconstruction of the anterior cruciate ligament helped to reduce varus-valgus as well as anterior- posterior instability. Varus-valgus rotation was further decreased from 2.2 to 1.3 times the control values after repair of the medial collateral ligament, but anterior-posterior knee stability did not change. A post hoc test of the power of our statistical tests, based on the most conservative parameter (anterior- posterior translation), revealed that a larger sample size may be needed to detect an effect of repair of the medial collateral ligament on anterior-posterior in- stability of the knee. Although anterior-posterior and varus-valgus knee instability did not decrease with time, the varus-valgus rotation in the knees with a repaired medial collateral ligament was significantly less at each study time than that in the knees with a nonrepaired medial collateral ligament. This evident reduction in varus-valgus knee instability may, in turn, assist in healing of the medial collateral ligament.

The higher ultimate load in the repair group at 12 weeks may have been due to the larger cross-sectional area of the medial collateral ligament and to better healing at the tibial insertion site. In this study, the presence of sutures and the tissue reaction to them probably was responsible for the larger cross-sectional area. Although the cross-sectional area was larger, the quality of the tissue did not improve, as evidenced by the lack of difference between the modulus of the repaired and the nonrepaired medial collateral liga- ments. However, as indicated previously, the effect of repair of the medial collateral ligament on modulus may not have been detectable with the sample size of this study. In each time period, two of the repaired medial collateral ligaments failed within the midsub- stance, whereas all of the nonrepaired ligaments failed at the tibial insertion site. One possible explanation

I Orthop Res, Vol. 13, No. 3, 1995

448 K. OHNO ET AL.

for this significant difference in the mode of failure is that increased tension in the sutured medial collat- eral ligament may help in the healing of the tibia1 insertion site. Similarly, a previous study suggested that increased stress on the healing ligament following an isolated injury of the medial collateral ligament helped to improve the quality of the healed femur- medial collateral ligament-tibia complex (11).

In the present study, we chose allografts as a substi- tute for the anterior cruciate ligament rather than au- tografts or synthetic grafts. The surgical technique of autografting, particularly in rabbits, is more difficult than that of allografting. Reconstruction of the ante- rior cruciate ligament using autografts has not led to a significant reduction of anterior-posterior instability of the knee immediately after surgery in rabbits (5) . Moreover, synthetic grafts, such as those made of Da- cron, wear with time. Twelve weeks after the use of Dacron grafts in rabbits, the anterior-posterior insta- bility of the reconstructed knee was 3.6 times greater than that immediately after surgery (9). In contrast, the use of allografts in the present study led to signif- icantly less anterior-posterior instability of the knee immediately after surgery, and the instability at 12 weeks was only 1.5 times greater.

Healing of the medial collateral ligament after a combined injury of the medial collateral and anterior cruciate ligaments differed from healing after an iso- lated injury of the medial collateral ligament. Using a rabbit model of an isolated injury of the medial col- lateral ligament, a previous study in our laboratory found that, at 12 weeks, the varus-valgus rotation of all knees in the repair and nonrepair groups was 1.3 times that of the knees in the control group and that the stiffness and ultimate load of the femur-medial collateral ligament-tibia complexes did not differ be- tween the experimental groups (28). With a combined injury of the ligaments in the present study, the varus- valgus rotations in the repair and nonrepair groups remained elevated and were 1.7 and 3.2 times, re- spectively, that of the values in the control group at 12 weeks. Furthermore, the ultimate load was signifi- cantly greater in the repair group than in the nonre- pair group. These differences in the healing of the medial collateral ligament may be attributed to differ- ences in the degree of instability of the knee. When the combined injury was treated with only recon- struction of the anterior cruciate ligament, anterior- posterior and varus-valgus stability did not return to normal, and the kinematics of the knee joint changed; such effects may impair healing of the medial collat- eral ligament. These results indicate that our method of reconstructing the anterior cruciate ligament could not restore normal function of that ligament and failed to effectively convert this combined injury to an isolated injury of the medial collateral ligament.

In this rabbit model of a combined injury of the medial collateral and anterior cruciate ligaments, re- pair of the ruptured medial collateral ligament im- proves the varus-valgus stability of the knee and the ultimate load of the femur-medial collateral ligament- tibia complex in the early phase of healing compared with no repair of the medial collateral ligament. How- ever, it should be noted that we cannot make direct clinical recommendations on the basis of the results in the present study, and further studies must be con- ducted to substantiate our findings.

Acknowledgment: This work was supported by National Insti- tutes of Health Grant AR41820 and the University of Pittsburgh Medical Center. The authors wish to thank Mr. Shawn Ward for his surgical assistance and 3M for the staples.

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