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Effect of soft tissue tension on measurements of coronal laxity in mobile-bearing total knee arthroplasty
J Orthop Sci (2005) 10:496–500 DOI 10.1007/s00776-005-0935-3
Original article Effect of soft tissue tension on measurements of coronal laxity in mobile-bearing total knee arthroplasty Yoshinori Ishii, Yoshikazu Matsuda, Hideo Noguchi, and Hiroshi Kiga Ishii Orthopaedic and Rehabilitation Clinic, 1089 Shimo-Oshi, Gyoda 361-0037, Japan
Abstract Background. The purpose of this study was to determine the effect of intraoperative coronal laxity in total knee arthroplasty on the postoperative condition. Methods. We conducted stress arthrometric studies using a Telos arthrometer on 40 knees in 36 patients. Both posterior cruciate ligament-retaining (PCLR) prostheses and posterior cruciate-sacrificing (PCLS) prostheses were placed in 20 knees respectively. All of the TKA procedures were judged clinically successful (Hospital for Special Surgery scores: PCLR 92 ± 3 points, PCLS 91 ± 4 points). Laxities were measured under spinal anesthesia (immediately postoperatively) and 6 months postoperatively. Results. PCLR prostheses had an average of 2.9° ± 1.8° and 3.0° ± 1.2° in abduction and 4.4° ± 2.8° and 3.6° ± 1.5° in adduction under anesthesia and the postoperative condition. PCLS prostheses had average laxities of 3.8° ± 1.4° and 3.5° ± 0.9° in abduction and 4.6° ± 3.8° and 4.0° ± 1.7° in adduction. There were no significant differences between them. Conclusions. The findings suggest that surgeons should emphasize the achievement of suitable laxity under anesthesia to ensure the success of total knee arthroplasty.
Introduction One of the major goals of a mobile-bearing knee design is to promote load sharing throughout the relative displacement of the tibial and femoral components.1 Simply stated, these designs allow the torque and shear forces that occur during weight-bearing, such as in normal gaits or deep-knee bending, to be transferred via displacement to soft tissues in a manner similar to that
Offprint requests to: Y. Ishii Received: March 14, 2005 / Accepted: June 23, 2005
of the normal knee. Tissues, unlike the inert prosthesis, have the capacity to respond and remodel when challenged by increased activity as the pain-free knee is rehabilitated. Ultimately, load sharing with these devices may contribute to reducing articular wear by decreasing the joint load. Therefore, soft tissue involvement should generally be encouraged to decrease dependence on the intrinsic constraints afforded by the condylar geometry. Information on soft tissue tension thus becomes indispensable for the determining the kinematics and clinical results after mobile-bearing total knee arthroplasty (TKA). Knee joint laxities are defined in three planes: horizontal (internal/external), sagittal (anterior/posterior), and coronal (abduction/adduction). Several studies focusing on coronal plane laxity have documented the specific advantages of posterior cruciate ligament (PCL)-retaining designs over PCL-substituting designs by using various methods postoperatively, including electrogoniometry,2 fluoroscopy,3 and arthrometry.4 Factors affecting the amount of coronal plane laxity can be divided into two categories. The first is soft tissue factors, such as the accuracy of ligamentous balancing during surgery, the direction of the resulting muscle force across the knee, and the dynamic stabilizing effect of the supporting musculature.5 The other group of factors concerns the hard tissues, such as the component geometry, the position of the components, and the rotation of the femur relative to the tibia (abduction/adduction in early to mid flexion).6 The effect of anesthesia, which would tend to exclude the contributions of soft tissue tension, on knee joint laxity has mainly been studied as it applies to repair of the anterior cruciate ligament. Those studies have determined the cutoff point of ACL injury,7,8 diagnostic accuracy,9,10 or the accuracy of the KT-1000 arthrometer.11 The purpose of the present study was to determine the effect of intraoperative coronal laxity in TKA on the postoperative condition.
Y. Ishii et al.: Coronal laxity after TKA
497
Table 1. Patient characteristics Parameter Knees/patients Sex (male/female) Age (years), mean and range Flexion, mean ± SD HSS score ± SD Coronal alignmenta, mean ± SD
PCLR
PCLS
20/20 5/15 72 (56–83) 118° ± 15° 92 ± 3 5.8° ± 3.2°
20/20 2/18 74 (56–85) 119° ± 17° 91 ± 4 5.7° ± 3.0°
PCLR, posterior cruciate ligament-retaining prosthesis; PCLS, posterior cruciate ligament-sacrificing prosthesis; HSS, Hospital for Special Surgery Radiographic analysis was performed using the Knee Society Radiographic Assessment12 a Valgus
A Materials and methods The knees of 36 patients (40 knees) who underwent TKA with LCS total knee systems (Depuy, Warsaw, IN, USA) were evaluated. The preoperative diagnosis for all of the patients was osteoarthritis. The clinical characteristics of the patients are summarized in Table 1. Twenty knees received meniscal bearing type (PCLretaining) prostheses and 20 knees received rotating platform type (PCL-sacrificing) prostheses. The surgeries were undertaken in random order. Because the two prosthetic designs have the same geometry in the coronal plane, any differences in laxity were considered attributable to the effects of the PCL. All of the TKA procedures were judged clinically successful [Hospital for Special Surgery (HSS) scores13: PCLR 92 ± 3 points, PCLS 91 ± 4 points), with no ligamentous instability or pain 6 months postoperatively. All surgeries were performed by a single surgeon (Y.I.) under spinal anesthesia (dibucaine hydrochloride, mean 6.0 ± 0.5 mg) using a standardized technique, including the necessary soft tissue release to obtain proper balances. The proper intraoperative coronal plane laxity was confirmed manually, although no intraoperative quantitative evaluation was performed. In all knees, the femoral components were fixed without cement, and the tibial components were fixed with cement. No revision knee replacements or conversions from high tibial osteotomy were included in the study. Informed consent, including a description of the protocol and the potential arthrometer-related complications, was obtained from all of the patients. The coronal plane laxity on abduction and adduction was measured using a Telos arthrometer (Fa Telos; Medizinisch-Technische, Griesheim, Germany) using standard technique. For the abduction/adduction stress test, 150 N was applied just above the joint on the lateral or medial femoral condyle (Fig. 1) with the knee in 0°– 20° flexion. The measurements were made immediately after surgery, with the patient still under anesthesia
B Fig. 1. Abduction and adduction stress were applied using a Telos arthrometer with 150 N. Abduction (A) and adduction (B) stress views
(Fig. 2A,B) and 6 months postoperatively (Fig. 2C,D) for each subject. The duration of surgery was 62 ± 12 min (range 48–75 min) for the PCLR group and 60 ± 11 min (range 49–74 min) for the PCLS group. The amount of time from the administration of anesthesia to the measurements were 66 ± 12 min (range 52–80 min) for the PCLR group and 64 ± 13 min (range 60–78 min) for the PCLS group, respectively. To minimize intertester variation, one observer (Y.M.) performed all of the tests. The intrasubject error was less than 1°. The paired t-test and nonpaired t-test were used to compare the two groups for statistical analysis.
Results Tables 2 and 3 present the mean abduction and adduction for each group at each time of measurement. For knees that underwent PCLR arthroplasty, there were no significant differences between the measurements made under anesthesia and those made 6 months
498
Y. Ishii et al.: Coronal laxity after TKA
B
A
D
C Table 2. Magnitudes of abduction and adduction in the PCLR group PCLR
Abduction
Adduction
Under anesthesia Six months postoperatively P
2.9° ± 1.8° 3.0° ± 1.2° 0.8127
4.4° ± 2.8° 3.6° ± 1.5° 0.2050
Table 3. Magnitudes of abduction and adduction in the PCLS group PCLS
Abduction
Adduction
Under anesthesia Six months postoperatively P
3.8° ± 1.4° 3.5° ± 0.9° 0.2012
4.6° ± 3.8° 4.0° ± 1.7° 0.3907
postoperatively for either abduction (2.9° ± 1.8° and 3.0° ± 1.2°, respectively) or adduction (4.4° ± 2.8° and 3.6° ± 1.5°, respectively). Similarly, for knees that underwent PCLS arthroplasty, there were no significant
Fig. 2. Stress radiographs obtained under anesthesia (A abduction; B adduction) and 6 months postoperatively (C abduction; D adduction) are shown for the posterior cruciate ligament-retaining arthroplasty
differences between the measurements made under anesthesia and those made 6 months postoperatively for either abduction (3.8° ± 1.4° and 3.5° ± 0.9°, respectively) or adduction (4.6° ± 3.8° and 4.0° ± 1.7°, respectively). HSS in both groups did not show significant differences (P = 0.4287) under the current laxities.
Discussion Ligamentous competence has a greater role in the mobile-bearing knee than in the fixed-bearing knee.14 Reasonable anteroposterior laxity to absorb shear stress and balanced abduction/adduction laxity to minimize micromotion between the implant and bone might contribute to the clinical success of TKA. Additionally, soft tissue tension is thought to be an important factor influencing the kinematics after TKA, along with the structures and the geometry of the prosthesis place-
Y. Ishii et al.: Coronal laxity after TKA
ment. Because anomalous kinematics following TKA are thought to contribute to catastrophic polyethylene wear, achieving unstrained kinematics after arthroplasty is one way to improve the longevity of the prosthesis. Therefore, favorable soft tissue laxities for TKA have been reported for the sagittal15,16 and coronal17–19 directions, or both.4 However, most of these studies have evaluated the joint laxity only postoperatively. There have been no studies, to our knowledge, that have reported the degree to which intraoperative measurements of coronal plane laxity, made under anesthesia, might reflect the postoperative laxity after TKA. The results of the current study indicate that measuring coronal plane laxity under anesthesia is a reliable indicator of postoperative laxity in extension. This correlation should be kept in mind when adjusting the joint laxity intraoperatively and during preparation of both the femur and the tibia. That is, the intraoperative laxities should not be calibrated to be either tight or loose in an attempt to compensate for the effects of anesthesia but, rather, should be adjusted to the optimal condition the surgeon would choose as a final postoperative outcome. To our knowledge, there is no established normal range for coronal plane laxity as measured by a mechanical arthrometer in knees with prosthetic replacements. Recently, two studies reported favorable laxity in the coronal plane. Draganich and Pottenger20 reported that the two radius area contact (TRAC) posterior stabilized mobile-bearing prosthesis showed 3° abduction and 4° adduction at 20° of flexion using a three-dimensional knee-laxity testing system. Ishii et al.18 performed stress arthrometric studies on 77 knees that had received Genesis I prostheses for TKA (Smith & Nephew Richards, Memphis, TN, USA) with a minimum follow-up of 5 years. They reported that the mean values for abduction and adduction were 4.8° and 4.5° with PCLR prostheses, respectively, and 4.6° and 4.0° with PCLS prostheses. Matsuda and Ishii4 found that, for low contact stress TKA, the mean values for abduction and adduction were 4.0° and 3.5° with a PCLR prostheses, respectively, and 3.8° and 3.9° with a PCLS prostheses. Kuster et al.19 defined a knee as “tight” if it opened less than 4° and “lax” if it opened 4° or more, as measured on manual stress radiographs after bilateral TKA. The authors reported an average 4.0° of abduction and 4.3° of adduction. The laxity measurements in the current study fell within the same range as those described in these prior reports. In the current study, neither the PCLR nor the PCLS knee designs revealed any differences in laxity measurements made under anesthesia or 6 months postoperatively. The effect of the retained PCL on coronal plane laxity seemed to be minimal with the prosthesis
499
design used in this study. There are three possible reasons for this result. First, the PCL might have less effect on coronal laxity as a second stabilizer in the prosthetic knee than it does in normal knees or in anterior cruciate ligament-deficient knees. Burstein and Wright21 examined the varus and valgus stabilizing structures in the knee and showed that the primary stabilizers were the collateral ligaments, whereas the cruciate ligaments contribute only 25% of the varus-valgus stability of the knee compared with the collateral ligaments. Grood et al.22 reported that the cruciates contributed 14.8% to medial restraint and 22.2% to lateral constraint at 5° of knee flexion. Second, the extended position of the knee during measurement might mask differences in laxity. Resection of the PCL increased both the extension and flexion gaps and had a more substantial effect on the laxity measured with the knee in flexion.23 Girgis et al.24 and Lew and Lewis25 also reported that the PCL tightens in flexion. Third, the geometry of the prostheses used in the current study had highly conforming articulations with the knee in extension,26 which might have exerted a much greater effect on stability than did retention of the PCL. This study had some limitations because the measurements of laxity were obtained only in extension owing to the characteristics of the arthrometer used. We are well aware of the importance of evaluating laxity over the entire range of knee flexion to confirm proper tension in the coronal direction. In addition, it would be informative to examine changes in PCL laxity attributable to degeneration or attenuation when using the PCLR design because the current study measured laxity at only 6 months after surgery.
Conclusions Measuring coronal-plane laxity in extension under anesthesia appears to be a useful indicator of postoperative results. This finding suggests that surgeons should emphasize achieving suitable laxity under anesthesia, both during bone preparation and soft tissue balancing, to ensure the success of TKA. The authors did not receive and will not receive any benefits and out side funding from any commercial party related directly or indirectly to the subject of this article.
References 1. Callaghan JJ, Insall JN, Greenwald AS, Dennis DA, Komistek RD, Murray DW, et al. Mobile-bearing knee replacement: concepts and results. J Bone Joint Surg Am 2000;82:1020–41. 2. Ishii Y, Terajima K, Koga Y, Takahashi HE, Bechtold JE, Gustilo RB. Gait analysis after total knee arthroplasty: comparison of
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4. 5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Y. Ishii et al.: Coronal laxity after TKA posterior cruciate retention and substitution. J Orthop Sci 1998;3: 310–7. Dennis DA, Komistek RD, Walker SA, Cheal EJ, Stiehl JB. Femoral condylar lift-off in vivo in total knee arthroplasty. J Bone Joint Surg Br 2001;83:33–9. Matsuda Y, Ishii Y. In vivo laxity in low contact stress mobilebearing prostheses. Clin Orthop 2004;419:138–43. Olmstead TG, Wevers HW, Bryant JT, Gouw GJ. Effect of muscular activity on valgus/ varus laxity and stiffness of the knee. J Biomech 1986;19:565–77. Romero J, Duronio JF, Sohrabi A, Alexander N, MacWilliams BA, Jones LC, et al. Varus and valgus flexion laxity of total knee alignment methods in loaded cadaveric knees. Clin Orthop 2002; 394:243–53. Daniel DM, Stone ML, Sachs R, Malcom L. Instrumented measurement of anterior knee laxity in patients with acute anterior cruciate ligament disruption. Am J Sports Med 1985;13:401–7. Highgenboten CL, Jackson AW, Jansson KA, Meske NB. KT1000 arthrometer: conscious and unconscious test results using 15, 20, and 30 pounds of force. Am J Sports Med 1992;20:450–4. Dahlstedt LJ, Dalen N. Knee laxity in cruciate ligament injury: value of examination under anesthesia. Acta Orthop Scand 1989; 60:181–4. Katz JW, Fingeroth RJ. The diagnostic accuracy of ruptures of the anterior cruciate ligament comparing the Lachman test, the anterior drawer sign, and the pivot shift test in acute and chronic knee injuries. Am J Sports Med 1986;14:88–91. Sernert N, Kartus J, Köhler K, Ejerhed L, Karlsson J. Evaluation of the reproducibility of the KT-1000 arthrometer. Scand J Med Sci Sports 2001;11:120–5. Ewald FC. The Knee Society total knee arthroplasty roentgenographic evaluation and scoring system. Clin Orthop 1989;248:9– 12. Alicea J. Scoring systems and their validation for the arthritic knee. In: Insall JN, Scott WN, editors. Surgery of the knee. Vol 2, 3rd edn. New York: Churchill Livingstone; 2001. p 1507–15. Vertullo CJ, Easley ME, Scott WN, Insall JN. Mobile bearings in primary knee arthroplasty. J Am Acad Orthop Surg 2001;9:355– 64.
15. Ishii Y, Matsuda Y, Ishii R, Sakata S, Omori G. Sagittal laxity in vivo after total knee arthroplasty. Arch Orthop Trauma Surg. 2005;125:249–53. 16. Warren JP, Olanlokun TK, Cobb AG, Walker PS, Iverson BF. Laxity and function in knee replacements: a comparative study of three prosthetic designs. Clin Orthop 1994;305:200–8. 17. Edwards E, Miller J, Chan KH. The effect of postoperative collateral ligament laxity in total knee arthroplasty. Clin Orthop 1988; 236:44–51. 18. Ishii Y, Matsuda Y, Ishii R, Sakata S, Omori G. Coronal laxity in extension in vivo after total knee arthroplasty. J Orthop Sci 2003; 8:538–42. 19. Kuster MS, Bitschnau B, Votruda T. Influence of collateral ligament laxity on patient satisfaction after total knee arthroplasty: a comparative bilateral study. Arch Orthop Trauma Surg 2004;124: 415–7. 20. Draganich LF, Pottenger LA. The TRAC PS mobile-bearing prosthesis: design rationale and in vivo 3-dimensional laxity. J Arthroplasty 2000;15:102–12. 21. Burstein AH, Wright TM. Basic biomechanics. In: Insall JN, Scott WN, editors. Surgery of the knee. Vol 1, 3rd edn. New York: Churchill Livingstone; 2001. p. 215–31. 22. Grood ES, Noyes FR, Butler DL, Suntay WJ. Ligamentous and capsular restraints preventing straight medial and lateral laxity in intact human cadaver knees. J Bone Joint Surg Am 1981;63:1257– 69. 23. Matsueda M, Gengerke TR, Murphy M, Lew WD, Gustilo RB. Soft tissue release in total knee arthroplasty: cadaver study using knees without deformities. Clin Orthop 1999;366:264–73. 24. Girgis FG, Marshall JL, Al Monejem ARS. The cruciate of the knee joint: anatomical, functional and experimental analysis. Clin Orthop 1975;106:216–31. 25. Lew WD, Lewis JL. The effect of knee-prosthesis geometry on cruciate ligament mechanics during flexion. J Bone Joint Surg Am 1982;64:734–86. 26. Pappas MJ. Engineering design of the LCS replacement. In: Hamelynck KJ, Stiehl JB, editors. LCS: mobile bearing knee arthroplasty: 25 years of worldwide experience. Berlin: Springer; 2002. p. 39–52.
Original article Effect of soft tissue tension on measurements of coronal laxity in mobile-bearing total knee arthroplasty Yoshinori Ishii, Yoshikazu Matsuda, Hideo Noguchi, and Hiroshi Kiga Ishii Orthopaedic and Rehabilitation Clinic, 1089 Shimo-Oshi, Gyoda 361-0037, Japan
Abstract Background. The purpose of this study was to determine the effect of intraoperative coronal laxity in total knee arthroplasty on the postoperative condition. Methods. We conducted stress arthrometric studies using a Telos arthrometer on 40 knees in 36 patients. Both posterior cruciate ligament-retaining (PCLR) prostheses and posterior cruciate-sacrificing (PCLS) prostheses were placed in 20 knees respectively. All of the TKA procedures were judged clinically successful (Hospital for Special Surgery scores: PCLR 92 ± 3 points, PCLS 91 ± 4 points). Laxities were measured under spinal anesthesia (immediately postoperatively) and 6 months postoperatively. Results. PCLR prostheses had an average of 2.9° ± 1.8° and 3.0° ± 1.2° in abduction and 4.4° ± 2.8° and 3.6° ± 1.5° in adduction under anesthesia and the postoperative condition. PCLS prostheses had average laxities of 3.8° ± 1.4° and 3.5° ± 0.9° in abduction and 4.6° ± 3.8° and 4.0° ± 1.7° in adduction. There were no significant differences between them. Conclusions. The findings suggest that surgeons should emphasize the achievement of suitable laxity under anesthesia to ensure the success of total knee arthroplasty.
Introduction One of the major goals of a mobile-bearing knee design is to promote load sharing throughout the relative displacement of the tibial and femoral components.1 Simply stated, these designs allow the torque and shear forces that occur during weight-bearing, such as in normal gaits or deep-knee bending, to be transferred via displacement to soft tissues in a manner similar to that
Offprint requests to: Y. Ishii Received: March 14, 2005 / Accepted: June 23, 2005
of the normal knee. Tissues, unlike the inert prosthesis, have the capacity to respond and remodel when challenged by increased activity as the pain-free knee is rehabilitated. Ultimately, load sharing with these devices may contribute to reducing articular wear by decreasing the joint load. Therefore, soft tissue involvement should generally be encouraged to decrease dependence on the intrinsic constraints afforded by the condylar geometry. Information on soft tissue tension thus becomes indispensable for the determining the kinematics and clinical results after mobile-bearing total knee arthroplasty (TKA). Knee joint laxities are defined in three planes: horizontal (internal/external), sagittal (anterior/posterior), and coronal (abduction/adduction). Several studies focusing on coronal plane laxity have documented the specific advantages of posterior cruciate ligament (PCL)-retaining designs over PCL-substituting designs by using various methods postoperatively, including electrogoniometry,2 fluoroscopy,3 and arthrometry.4 Factors affecting the amount of coronal plane laxity can be divided into two categories. The first is soft tissue factors, such as the accuracy of ligamentous balancing during surgery, the direction of the resulting muscle force across the knee, and the dynamic stabilizing effect of the supporting musculature.5 The other group of factors concerns the hard tissues, such as the component geometry, the position of the components, and the rotation of the femur relative to the tibia (abduction/adduction in early to mid flexion).6 The effect of anesthesia, which would tend to exclude the contributions of soft tissue tension, on knee joint laxity has mainly been studied as it applies to repair of the anterior cruciate ligament. Those studies have determined the cutoff point of ACL injury,7,8 diagnostic accuracy,9,10 or the accuracy of the KT-1000 arthrometer.11 The purpose of the present study was to determine the effect of intraoperative coronal laxity in TKA on the postoperative condition.
Y. Ishii et al.: Coronal laxity after TKA
497
Table 1. Patient characteristics Parameter Knees/patients Sex (male/female) Age (years), mean and range Flexion, mean ± SD HSS score ± SD Coronal alignmenta, mean ± SD
PCLR
PCLS
20/20 5/15 72 (56–83) 118° ± 15° 92 ± 3 5.8° ± 3.2°
20/20 2/18 74 (56–85) 119° ± 17° 91 ± 4 5.7° ± 3.0°
PCLR, posterior cruciate ligament-retaining prosthesis; PCLS, posterior cruciate ligament-sacrificing prosthesis; HSS, Hospital for Special Surgery Radiographic analysis was performed using the Knee Society Radiographic Assessment12 a Valgus
A Materials and methods The knees of 36 patients (40 knees) who underwent TKA with LCS total knee systems (Depuy, Warsaw, IN, USA) were evaluated. The preoperative diagnosis for all of the patients was osteoarthritis. The clinical characteristics of the patients are summarized in Table 1. Twenty knees received meniscal bearing type (PCLretaining) prostheses and 20 knees received rotating platform type (PCL-sacrificing) prostheses. The surgeries were undertaken in random order. Because the two prosthetic designs have the same geometry in the coronal plane, any differences in laxity were considered attributable to the effects of the PCL. All of the TKA procedures were judged clinically successful [Hospital for Special Surgery (HSS) scores13: PCLR 92 ± 3 points, PCLS 91 ± 4 points), with no ligamentous instability or pain 6 months postoperatively. All surgeries were performed by a single surgeon (Y.I.) under spinal anesthesia (dibucaine hydrochloride, mean 6.0 ± 0.5 mg) using a standardized technique, including the necessary soft tissue release to obtain proper balances. The proper intraoperative coronal plane laxity was confirmed manually, although no intraoperative quantitative evaluation was performed. In all knees, the femoral components were fixed without cement, and the tibial components were fixed with cement. No revision knee replacements or conversions from high tibial osteotomy were included in the study. Informed consent, including a description of the protocol and the potential arthrometer-related complications, was obtained from all of the patients. The coronal plane laxity on abduction and adduction was measured using a Telos arthrometer (Fa Telos; Medizinisch-Technische, Griesheim, Germany) using standard technique. For the abduction/adduction stress test, 150 N was applied just above the joint on the lateral or medial femoral condyle (Fig. 1) with the knee in 0°– 20° flexion. The measurements were made immediately after surgery, with the patient still under anesthesia
B Fig. 1. Abduction and adduction stress were applied using a Telos arthrometer with 150 N. Abduction (A) and adduction (B) stress views
(Fig. 2A,B) and 6 months postoperatively (Fig. 2C,D) for each subject. The duration of surgery was 62 ± 12 min (range 48–75 min) for the PCLR group and 60 ± 11 min (range 49–74 min) for the PCLS group. The amount of time from the administration of anesthesia to the measurements were 66 ± 12 min (range 52–80 min) for the PCLR group and 64 ± 13 min (range 60–78 min) for the PCLS group, respectively. To minimize intertester variation, one observer (Y.M.) performed all of the tests. The intrasubject error was less than 1°. The paired t-test and nonpaired t-test were used to compare the two groups for statistical analysis.
Results Tables 2 and 3 present the mean abduction and adduction for each group at each time of measurement. For knees that underwent PCLR arthroplasty, there were no significant differences between the measurements made under anesthesia and those made 6 months
498
Y. Ishii et al.: Coronal laxity after TKA
B
A
D
C Table 2. Magnitudes of abduction and adduction in the PCLR group PCLR
Abduction
Adduction
Under anesthesia Six months postoperatively P
2.9° ± 1.8° 3.0° ± 1.2° 0.8127
4.4° ± 2.8° 3.6° ± 1.5° 0.2050
Table 3. Magnitudes of abduction and adduction in the PCLS group PCLS
Abduction
Adduction
Under anesthesia Six months postoperatively P
3.8° ± 1.4° 3.5° ± 0.9° 0.2012
4.6° ± 3.8° 4.0° ± 1.7° 0.3907
postoperatively for either abduction (2.9° ± 1.8° and 3.0° ± 1.2°, respectively) or adduction (4.4° ± 2.8° and 3.6° ± 1.5°, respectively). Similarly, for knees that underwent PCLS arthroplasty, there were no significant
Fig. 2. Stress radiographs obtained under anesthesia (A abduction; B adduction) and 6 months postoperatively (C abduction; D adduction) are shown for the posterior cruciate ligament-retaining arthroplasty
differences between the measurements made under anesthesia and those made 6 months postoperatively for either abduction (3.8° ± 1.4° and 3.5° ± 0.9°, respectively) or adduction (4.6° ± 3.8° and 4.0° ± 1.7°, respectively). HSS in both groups did not show significant differences (P = 0.4287) under the current laxities.
Discussion Ligamentous competence has a greater role in the mobile-bearing knee than in the fixed-bearing knee.14 Reasonable anteroposterior laxity to absorb shear stress and balanced abduction/adduction laxity to minimize micromotion between the implant and bone might contribute to the clinical success of TKA. Additionally, soft tissue tension is thought to be an important factor influencing the kinematics after TKA, along with the structures and the geometry of the prosthesis place-
Y. Ishii et al.: Coronal laxity after TKA
ment. Because anomalous kinematics following TKA are thought to contribute to catastrophic polyethylene wear, achieving unstrained kinematics after arthroplasty is one way to improve the longevity of the prosthesis. Therefore, favorable soft tissue laxities for TKA have been reported for the sagittal15,16 and coronal17–19 directions, or both.4 However, most of these studies have evaluated the joint laxity only postoperatively. There have been no studies, to our knowledge, that have reported the degree to which intraoperative measurements of coronal plane laxity, made under anesthesia, might reflect the postoperative laxity after TKA. The results of the current study indicate that measuring coronal plane laxity under anesthesia is a reliable indicator of postoperative laxity in extension. This correlation should be kept in mind when adjusting the joint laxity intraoperatively and during preparation of both the femur and the tibia. That is, the intraoperative laxities should not be calibrated to be either tight or loose in an attempt to compensate for the effects of anesthesia but, rather, should be adjusted to the optimal condition the surgeon would choose as a final postoperative outcome. To our knowledge, there is no established normal range for coronal plane laxity as measured by a mechanical arthrometer in knees with prosthetic replacements. Recently, two studies reported favorable laxity in the coronal plane. Draganich and Pottenger20 reported that the two radius area contact (TRAC) posterior stabilized mobile-bearing prosthesis showed 3° abduction and 4° adduction at 20° of flexion using a three-dimensional knee-laxity testing system. Ishii et al.18 performed stress arthrometric studies on 77 knees that had received Genesis I prostheses for TKA (Smith & Nephew Richards, Memphis, TN, USA) with a minimum follow-up of 5 years. They reported that the mean values for abduction and adduction were 4.8° and 4.5° with PCLR prostheses, respectively, and 4.6° and 4.0° with PCLS prostheses. Matsuda and Ishii4 found that, for low contact stress TKA, the mean values for abduction and adduction were 4.0° and 3.5° with a PCLR prostheses, respectively, and 3.8° and 3.9° with a PCLS prostheses. Kuster et al.19 defined a knee as “tight” if it opened less than 4° and “lax” if it opened 4° or more, as measured on manual stress radiographs after bilateral TKA. The authors reported an average 4.0° of abduction and 4.3° of adduction. The laxity measurements in the current study fell within the same range as those described in these prior reports. In the current study, neither the PCLR nor the PCLS knee designs revealed any differences in laxity measurements made under anesthesia or 6 months postoperatively. The effect of the retained PCL on coronal plane laxity seemed to be minimal with the prosthesis
499
design used in this study. There are three possible reasons for this result. First, the PCL might have less effect on coronal laxity as a second stabilizer in the prosthetic knee than it does in normal knees or in anterior cruciate ligament-deficient knees. Burstein and Wright21 examined the varus and valgus stabilizing structures in the knee and showed that the primary stabilizers were the collateral ligaments, whereas the cruciate ligaments contribute only 25% of the varus-valgus stability of the knee compared with the collateral ligaments. Grood et al.22 reported that the cruciates contributed 14.8% to medial restraint and 22.2% to lateral constraint at 5° of knee flexion. Second, the extended position of the knee during measurement might mask differences in laxity. Resection of the PCL increased both the extension and flexion gaps and had a more substantial effect on the laxity measured with the knee in flexion.23 Girgis et al.24 and Lew and Lewis25 also reported that the PCL tightens in flexion. Third, the geometry of the prostheses used in the current study had highly conforming articulations with the knee in extension,26 which might have exerted a much greater effect on stability than did retention of the PCL. This study had some limitations because the measurements of laxity were obtained only in extension owing to the characteristics of the arthrometer used. We are well aware of the importance of evaluating laxity over the entire range of knee flexion to confirm proper tension in the coronal direction. In addition, it would be informative to examine changes in PCL laxity attributable to degeneration or attenuation when using the PCLR design because the current study measured laxity at only 6 months after surgery.
Conclusions Measuring coronal-plane laxity in extension under anesthesia appears to be a useful indicator of postoperative results. This finding suggests that surgeons should emphasize achieving suitable laxity under anesthesia, both during bone preparation and soft tissue balancing, to ensure the success of TKA. The authors did not receive and will not receive any benefits and out side funding from any commercial party related directly or indirectly to the subject of this article.
References 1. Callaghan JJ, Insall JN, Greenwald AS, Dennis DA, Komistek RD, Murray DW, et al. Mobile-bearing knee replacement: concepts and results. J Bone Joint Surg Am 2000;82:1020–41. 2. Ishii Y, Terajima K, Koga Y, Takahashi HE, Bechtold JE, Gustilo RB. Gait analysis after total knee arthroplasty: comparison of
500
3.
4. 5.
6.
7.
8.
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