- Email: [email protected]
In vivo knee laxity in flexion and extension: A radiographic study in 30 older healthy subjects
Available online at www.sciencedirect.com
The Knee 15 (2008) 45 – 49
In vivo knee laxity in flexion and extension: A radiographic study in 30 older healthy subjects P.J.C. Heesterbeek a,⁎, N. Verdonschot b , A.B. Wymenga c a b
Sint Maartenskliniek, Department of Research, Development & Education, Postbox 9011, 6500 GM Nijmegen, The Netherlands Radboud University Nijmegen Medical Centre, Orthopaedic Research Lab, Postbox 9101, 6500 HB Nijmegen, The Netherlands c Sint Maartenskliniek, Department of Orthopaedics, Postbox 9011, 6500 GM Nijmegen, The Netherlands Received 2 May 2007; received in revised form 13 September 2007; accepted 25 September 2007
Abstract In order to determine how “tight” a total knee prosthesis should be implanted, it is important to know the amount of laxity in a healthy knee. The objective of this study was to determine knee laxity in extension and flexion in healthy, non-arthritic knees of subjects similar in age to patients undergoing a total knee arthroplasty and to provide guidelines for the orthopaedic surgeon in his attempt to restore the stability of an osteoarthritic knee to normal. Thirty healthy subjects (15 male, 15 female), mean age 62 (SD 6.4) years, were included in the study. For each subject one, randomly selected, knee was stressed in extension and in 70° flexion (15 Nm). Varus and valgus laxity were measured on radiographs. The passive range of motion and active flexion was assessed. Mean valgus laxity in extension was 2.3° (SD 0.9, range 0.2°–4.1°). In extension mean varus laxity was 2.8° (SD 1.3, range 0.6°–5.4°). In flexion, mean valgus laxity was 2.5° (SD 1.5, range 0.0°–6.0°) and mean varus laxity was 3.1° (SD 2.0, range 0.1°–7.0°). Varus and valgus knee laxity in extension and in flexion were comparable. This study shows that the normal knee in this age group has an inherent degree of varus–valgus laxity. Whether the results of the present study can be used to optimise the total knee arthroplasty implantation technique requires further investigation. © 2007 Elsevier B.V. All rights reserved. Keywords: Varus laxity; Valgus laxity; Collateral ligaments; Normal knees; Radiographs
1. Introduction The main goals of a primary total knee arthroplasty are to relieve the patient's pain and to restore the knee joint kinematics and stability to a normal level. Correct positioning of the components and adequate soft-tissue balancing in extension and flexion are essential to achieve proper kinematics and stability [1,2]. Insall et al. (1976, 1985) described in his classic approach how to tension a knee in flexion using spreaders after ligamentous release in extension to align the leg [3,4]. However, the optimal amount of force on the ligament spreaders is unknown. Also unknown is how tight a knee should be when tested with spacers during implantation. A high force applied to the ligament capsule of the knee could result in a total ⁎ Corresponding author. Tel.: +31 24 3659628; fax: +31 24 3659154. E-mail address: [email protected] (P.J.C. Heesterbeek). 0968-0160/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2007.09.007
knee prosthesis being implanted “too tightly”. This might cause limited range of motion. On the other hand, a low amount of force applied to the ligaments could result in a knee with a high degree of instability. While post-operative range of motion might be greater it could be accompanied by instability problems in flexion [5] and may even lead to failure of the prosthesis [6]. In order to determine how tight the prosthetic knee should be balanced, it is important to know the laxity in a healthy, nonarthritic knee. Obviously this cannot be quantified following the procedure used during TKA implantation. Therefore, indirect methods such as stress X-rays are needed to quantify the laxity of healthy knees. In addition it is important to determine varus and valgus laxity separately, since the anatomy and, therefore, the kinematics of the medial and lateral side differ [7–9]. Stähelin et al. (2003) reported a radiographic technique to quantify in vivo varus and valgus knee joint laxity in flexion [10], but they did not report the values of laxity found in their study population.
46
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
Fig. 1. Flexilax: assessment of varus and valgus laxity of the knee in 70° flexion. A. Lateral view, B. view from above.
Within the diversity of reports available, most of which focused on varus–valgus laxity in extension or 20° of flexion, we found no quantitative data for varus–valgus laxity in flexion for the healthy older knee. The laxity in older subjects is of great clinical relevance because this would apply to most TKA patients. The objective of this study was to quantify the normal values for varus and valgus knee laxity in extension and flexion in healthy older subjects, thereby providing guidelines for the orthopaedic surgeon to restore the stability in the osteoarthritic knee and to optimise the total knee implantation techniques. 2. Methods 2.1. Subjects Before starting the study, a power calculation was carried out, and 28.7 patients were needed. The standard deviation and accuracy from a previous pilot study were 4.1° and 1.5°, respectively, with α set at 0.05 and β at 0.80. In total, 30 healthy subjects participated in this study (average age 62 (SD 6.4) years, 15 male, 15 female). The subjects were selected through an advertisement in the waiting room of our orthopaedic clinic and were included if their age was between 50–75, they had no osteo- or rheumatoid arthritis, no history of knee injury or knee complaints, and were able to walk for at least 1 h. Exclusion criteria were; previous hip arthrodesis or hip prosthesis, BMI N 35, knee flexion b 90°, treatment for osteoporosis, and readily visible varus or valgus leg alignment. For each subject one, randomly selected, knee was included in the study (15 left and 15 right knees). All subjects gave informed consent. The local ethics committee approved this study.
2.2. Measurements 2.2.1. Range of motion Passive range of motion of the knee (ROM) was measured with a goniometer by the research nurse. Active knee flexion was defined as the amount of maximum flexion that the subject could perform during stance without lifting the leg using the arms.
2.2.2. Varus–valgus laxity in extension Medial and lateral laxity of the knee in extension was assessed using the Telos device (Fa Telos, Medizinisch-Technische GmbH, Griesheim, Germany) with the subject lying in a supine position with leg muscles relaxed. The Telos device was applied with 15Nm load on the extended leg relative to the level of the joint line. While medial and lateral forces were applied, radiographs were obtained in the anteroposterior view. The direction of the X-rays was parallel to the tibia joint surface, centred on the middle of the femorotibial joint space. In addition, a radiograph without any medially or laterally applied force was made to determine the neutral position. 2.2.3. Varus–valgus laxity in flexion For medial and lateral flexion laxity, a custom made stress device was used to stress the knee and to produce reproducible measurements (Fig. 1). Before starting this study the reproducibility was thoroughly assessed. The 95% prediction limit (Bland and Altman) was 1.7° based on two radiographs of the same subject with a short interval between the radiographs and with the subject being repositioned (unpublished data). During the measurement the subject was in supine position with the lower leg on a plateau with the knee flexed in 70°. To minimize hip rotation the upper leg was strapped on a thigh support. Although measurement of flexion laxity is normally performed in 90° flexion, with this method radiographic examination in 90° knee flexion is not possible because the femur interferes with positioning of the radiographic equipment. The foot was held in an ankle–foot-orthosis. An external moment of 15 Nm was applied at the knee joint using 50 N on a pulley 0.30 m distal from the joint line (Fig. 1). The knee was stressed medially and laterally. Radiographs were made with the X-ray direction parallel to the tibia joint surface in the conditions varus, valgus or no moment applied. The subjects were instructed to relax their thigh muscles to minimize the role of the dynamic knee stabilizers. 2.2.4. Varus–valgus laxity calculation The angle between a tangent line on the femur condyles and a line through the deepest tibial joint surfaces was determined on the varus, valgus and neutral radiographs (Fig. 2) using the measurement tool within the radiographic database program (WEB1000 version 4.1, Agfa-Geveart AG, Rijswijk, The Netherlands). Valgus laxity was defined as the difference between the medial stress radiograph and the neutral radiograph, varus laxity as the difference between the lateral stress radiograph and the neutral radiograph. The measurements were made to the nearest 0.1°.
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
47
Fig. 2. Measurements of varus and valgus laxity on stress X-rays in flexion: neutral situation (A), valgus stress (B) and varus stress (C). 2.2.5. Statistical analyses Differences in varus and valgus laxity in extension and flexion for gender, were assessed using Student's t-test. The Shapiro–Wilk test was used to test for normal distribution. The association between age and varus–valgus laxity was investigated with a regression analysis as a regression of laxity on age with “age” as independent and “laxity” as dependent variable. Pearson's correlation coefficients (2-tailed) were calculated between varus and valgus laxity in extension, and varus and valgus laxity in flexion, and between varus laxity in extension and flexion and valgus laxity in extension and flexion. Results were presented as mean (SD) with range and 95% CI. All tests were performed with a level of significance of 0.05.
3.3. Varus–valgus laxity in flexion
Measurements could be performed on all subjects and no subject had to be excluded because of asymptomatic, radiographic arthritis.
The mean varus laxity in flexion was 3.1° (SD 2.0) and mean valgus laxity was 2.5° (SD 1.5) (Table 1). The Shapiro–Wilk test showed that both varus and valgus laxity in flexion were normally distributed (p = 0.1 and p = 0.6 for varus and valgus laxity, respectively). No difference in gender was found for varus laxity (p = 0.1) or valgus laxity (p = 0.2). There was a statistically significant association between age and varus laxity; varus laxity (degrees) = − 6.54 + 0.512⁎(degrees/years) (p = 0.004). There was no statistically significant association between age and valgus laxity. Pearson's correlation coefficient between varus and valgus laxity in flexion was 0.044 (p = 0.82). Between varus laxity in extension and flexion R = −0.043 (p = 0.82) and between valgus laxity in extension and flexion R = 0.41 (p = 0.025).
3.1. Range of motion
4. Discussion
Mean passive ROM was 132.3° (SD 7.3, range 115–140°). Mean active flexion was 115.7° (SD 15.2, range 80–140°). There were no significant correlations between either ROM or active flexion and varus– valgus laxity in extension or flexion.
The objective of this study was to quantify the amount of varus and valgus laxity in extension and flexion in the healthy older knee. There was a broad range for varus and valgus laxity, in extension as well as in flexion. This high inter-subject variability also seems to indicate that within individuals with functionally normal knees, the normal knee laxity can exist within a broad range. This study revealed a significant positive correlation between valgus laxity in extension and flexion, although only 17% of the amount of variability in valgus laxity in flexion could be explained by valgus laxity in extension. Overall the conclusion can be drawn that if a subject's knee is loose in extension, this does not mean that it also will be loose in flexion. The same applies for varus and valgus laxity. The data in the present study showed that higher age led to higher varus laxity in flexion. One other study also concluded that there was a (quite low; r = 0.29) correlation between varus–valgus laxity and age, as measured in extension [11]. In that study, older subjects had higher varus–valgus laxity than younger subjects. A point of discussion concerning the current study is the technical limitation imposed by not being able to measure in vivo varus–valgus laxity in 90° flexion with radiographs. Nevertheless, interpolation of data from Markolf et al. (1976) shows that the difference between laxity in 70° and 90° knee flexion with a
3. Results
3.2. Varus–valgus laxity in extension Mean varus laxity in extension was 2.8° (SD 1.3); mean valgus laxity in extension was 2.3° (SD 0.8) (Table 1). The Shapiro–Wilk test showed a normal distribution for both varus and valgus laxity (p = 0.8 and p = 0.4 for varus and valgus laxity, respectively). There was no significant gender difference for varus laxity (p = 0.2) or valgus laxity (p = 0.6). There was no statistically significant association between age and varus laxity or between age and valgus laxity. Pearson's correlation coefficient (R) between varus and valgus laxity in extension was −0.196 (p = 0.30).
Table 1 Descriptive data of varus and valgus laxity in extension and flexion
Extension varus laxity Extension valgus laxity Flexion varus laxity Flexion valgus laxity (n = 30).
Mean (°)
SD (°)
Range (°)
95% CI (°)
2.8 2.3 3.1 2.5
1.3 0.8 2.0 1.5
0.6–5.4 0.2–4.1 0.1–7.0 0.0–6.0
2.4–3.3 2.0–2.6 2.4–3.9 2.0–3.1
48
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
moment of 15 Nm is at the most 0.5° [12]. Although varus–valgus laxity should ideally be measured in 90° since the extension and flexion gap in a total knee prosthesis have an angle of 90°, we expect that our study will still be useful to provide guidelines for varus and valgus laxity in flexion and extension in TKA. Our in vivo findings approximate the in vitro knee laxities found during biomechanical tests of cadaver knee specimen by Markolf et al. [12] (Table 2). The cadaver study by Van Damme et al. (2005) found for extension similar laxity [13]. However, their results in flexion were higher than our in vivo results. A possible explanation for these differences might be the presence of muscular tension in our subjects that resulted in lower flexion laxity. Furthermore, their measurements were performed in mm, which when converted to degrees, could lower the laxity values. But they are still more than found in our study. Other in vivo knee laxity studies reported in the literature were performed with younger adults (Table 2). Okazaki et al. (2006) measured varus and valgus laxity on radiographs with knees stressed with Telos in 10° and 80° flexion in 19 to 59 years old individuals. Varus (lateral) laxity was significantly higher than valgus (medial) laxity in their study [14]. Possibly, the different measurement method may explain these differences. Another in vivo study in younger adults (aged 18 to 53) performed by Tokuhara et al. (2004) used MRI scans in 90° knee flexion with an unknown amount of load applied in 90° flexion. They also reported a higher mean varus (lateral) laxity than valgus (medial) laxity [15]. Both studies were performed in younger adults in a Japanese study population. Cultural differences, for example sitting position, and also anatomical differences between the Asian and Caucasian population might account for the higher lateral flexion laxity. Since the present study is the first to quantify varus and valgus laxity in extension and flexion in older, healthy (Caucasian) subjects, it remains difficult to compare the present results to the literature. In TKA the question is what amount of laxity is desired to obtain a good result. In the literature, studies report that a laxity of approximately 3° to 4° in extension for varus and valgus laxity is suitable to obtain good functional results in PCL retaining TKA [16–18]. Another study in bilateral TKA revealed that patients preferred the slacker knee above the tighter knee [19]. Drawing on the results of the present study, we would advise limiting the laxity in extension as well as flexion to less than 4°. Given the broad range of normal varus– valgus laxity it might be helpful to assess the varus–valgus Table 2 Literature overview Author
Markolf et al. [12] Van Damme et al. [13] Okazaki et al. [14] Tokuhara et al. [15]
In vivo/cadaver study
Extension laxity
Flexion laxity
Varus
Valgus
Varus
Valgus
Cadaver
1.5°
1.5°
5.5°
4.5°
Cadaver
3.1 mm (SD 0.8) 4.9° (SD 2.0) –
2.6 mm (SD 1.0) 2.4° (SD 1.6) –
8.1 mm (SD 1.0) 4.8° (SD 3.2) 6.7 mm (SD 1.9)
7.1 mm (SD 1.4) 1.7° (SD 1.4) 2.1 mm (SD 1.1)
In vivo, age 19–59 yr In vivo, age 18–53 yr
laxity in the healthy contralateral knee in case of TKP in a patient with unilateral osteoarthritis when deciding the correct ligament tension. Considering the advantages of our method to measure flexion laxity using radiographs, we can conclude that this method is a suitable repeatable method to evaluate flexion laxity in the natural knee. This flexion laxity measurement technique can also serve to analyse an unstable knee after total knee arthroplasty. This method enables a more precise measurement of varus and valgus laxity than can be obtained with physical examination. Clinical assessment of varus–valgus stability in the flexed knee is not always precise because tibiofemoral varus–valgus tilt can barely be distinguished from simultaneous hip rotation. Furthermore, the amount of joint opening is difficult to palpate and quantify, especially in heavier patients. Romero et al. investigated lateral flexion instability after TKA with an X-ray method which is comparable to the one used in the present study [20]. An interesting question is whether the presence of a certain amount of muscular tension may have influenced our results. Although the measurements were painless, the subjects did feel traction on their knee. These measurements were not performed in anaesthetized patients. During surgery there is certainly less muscular tension and knee laxity may be higher. However, the effect of muscular tension on knee laxity is unknown and should be investigated in future studies. The biomechanics of a replaced knee are different from those of a normal knee. It is therefore questionable if and how the normal values obtained in the present study can be applied to the total knee arthroplasty population. However, since this is still unknown, a good start may be to aim for a laxity that is within the normal range. With regard to the results reported and taking into consideration the repeatability errors that one can expect with this technique, we can provide the following recommendation to the surgeon: try to obtain a varus laxity in flexion between 0 and 7.1 degrees (95% confidence interval) and a valgus laxity between 0 and 5.5°. These values agree with the flexion laxity data of patients with good functioning total knee prostheses found by Romero et al. [20] In extension, the surgeon should try to obtain a varus laxity between 0.2 and 5.4° and a valgus laxity between 0.7 and 3.9°. Essentially, the results of this study indicate that the surgeon should balance the knee so that it does not have any varus–valgus laxity greater than 5.5 to 7°. Instead of using spacers or ‘feel’ to test varus–valgus laxity intra-operatively, the surgeon can use a navigation system in order to test varus–valgus laxity more accurately. When no navigation system is available, varus–valgus laxity can be estimated based on gap heights and goniometry. For example: for a varus laxity of 4°, the lateral gap has to be 3.5 mm greater than the medial gap when the distance between the contact points on the femoral condyles is 50 mm. From this study we can conclude that the range of varus– valgus laxity in extension and flexion in non-anaesthetised subjects without osteoarthritis is variable. Whether using these physiological laxity data for varus and valgus laxity during prosthetic implantation will affect clinical results in the long term is an important question and requires further investigation.
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
Acknowledgements The authors like to thank research nurse Henriëtte de Gouw for her assistance and P.G. Anderson for her editorial support, and the RMS Foundation, Switzerland for financial support in this study. References [1] Fehring TK. Rotational malalignment of the femoral component in total knee arthroplasty. Clin Orthop 2000;380:72–9. [2] Whiteside LA. Soft tissue balancing: the knee. J Arthroplasty 2002;17:23–7. [3] Insall J, Ranawat CS, Scott WN, Walker P. Total condylar knee replacement: preliminary report. Clin Orthop Relat Res 1976;120:149–54. [4] Insall JN, Binazzi R, Soudry M, Mestriner LA. Total knee arthroplasty. Clin Orthop Relat Res 1985;192:13–22. [5] Zalzal P, Papini M, Petruccelli D, de BJ, Winemaker MJ. An in vivo biomechanical analysis of the soft-tissue envelope of osteoarthritic knees. J Arthroplasty 2004;19:217–23. [6] Fehring TK, Odum S, Griffin WL, Mason JB, Nadaud M. Early failures in total knee arthroplasty. Clin Orthop Relat Res 2001:315–8. [7] Hill PF, Vedi V, Williams A, Iwaki H, Pinskerova V, Freeman MA. Tibiofemoral movement 2: the loaded and unloaded living knee studied by MRI. J Bone Joint Surg Br 2000;82:1196–8. [8] Iwaki H, Pinskerova V, Freeman MA. Tibiofemoral movement 1: the shapes and relative movements of the femur and tibia in the unloaded cadaver knee. J Bone Joint Surg Br 2000;82:1189–95. [9] Nakagawa S, Kadoya Y, Todo S, Kobayashi A, Sakamoto H, Freeman MA, et al. Tibiofemoral movement 3: full flexion in the living knee studied by MRI. J Bone Joint Surg Br 2000;82:1199–200.
49
[10] Stähelin T, Kessler O, Pfirrmann C, Jacob HA, Romero J. Fluoroscopically assisted stress radiography for varus–valgus stability assessment in flexion after total knee arthroplasty. J Arthroplasty 2003;18:513–5. [11] Sharma L, Lou C, Felson DT, Dunlop DD, Kirwan-Mellis G, Hayes KW, et al. Laxity in healthy and osteoarthritic knees. Arthritis Rheum 1999;42: 861–70. [12] Markolf KL, Mensch JS, Amstutz HC. Stiffness and laxity of the knee— the contributions of the supporting structures. A quantitative in vitro study. J Bone Joint Surg Am 1976;58:583–94. [13] Van Damme G, Defoort K, Ducoulombier Y, Van Glabbeek F, Bellemans J, Victor J. What should the surgeon aim for when performing computerassisted total knee arthroplasty? J Bone Joint Surg Am 2005;87(Suppl 2): 52–8. [14] Okazaki K, Miura H, Matsuda S, Takeuchi N, Mawatari T, Hashizume M, et al. Asymmetry of mediolateral laxity of the normal knee. J Orthop Sci 2006;11:264–6. [15] Tokuhara Y, Kadoya Y, Nakagawa S, Kobayashi A, Takaoka K. The flexion gap in normal knees. An MRI study. J Bone Joint Surg Br 2004;86:1133–6. [16] 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. [17] Matsuda Y, Ishii Y, Noguchi H, Ishii R. Effect of flexion angle on coronal laxity in patients with mobile-bearing total knee arthroplasty prostheses. J Orthop Sci 2005;10:37–41. [18] Ishii Y, Noguchi H, Matsuda Y, Takeda M, Walker SA, Komistek RD. Effect of knee laxity on in vivo kinematics of meniscal-bearing knee prostheses. Knee 2007;14:269–74. [19] Kuster MS, Bitschnau B, Votruba 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] Romero J, Stahelin T, Binkert C, Pfirrmann C, Hodler J, Kessler O. The clinical consequences of flexion gap asymmetry in total knee arthroplasty. J Arthroplasty 2007;22:235–40.
The Knee 15 (2008) 45 – 49
In vivo knee laxity in flexion and extension: A radiographic study in 30 older healthy subjects P.J.C. Heesterbeek a,⁎, N. Verdonschot b , A.B. Wymenga c a b
Sint Maartenskliniek, Department of Research, Development & Education, Postbox 9011, 6500 GM Nijmegen, The Netherlands Radboud University Nijmegen Medical Centre, Orthopaedic Research Lab, Postbox 9101, 6500 HB Nijmegen, The Netherlands c Sint Maartenskliniek, Department of Orthopaedics, Postbox 9011, 6500 GM Nijmegen, The Netherlands Received 2 May 2007; received in revised form 13 September 2007; accepted 25 September 2007
Abstract In order to determine how “tight” a total knee prosthesis should be implanted, it is important to know the amount of laxity in a healthy knee. The objective of this study was to determine knee laxity in extension and flexion in healthy, non-arthritic knees of subjects similar in age to patients undergoing a total knee arthroplasty and to provide guidelines for the orthopaedic surgeon in his attempt to restore the stability of an osteoarthritic knee to normal. Thirty healthy subjects (15 male, 15 female), mean age 62 (SD 6.4) years, were included in the study. For each subject one, randomly selected, knee was stressed in extension and in 70° flexion (15 Nm). Varus and valgus laxity were measured on radiographs. The passive range of motion and active flexion was assessed. Mean valgus laxity in extension was 2.3° (SD 0.9, range 0.2°–4.1°). In extension mean varus laxity was 2.8° (SD 1.3, range 0.6°–5.4°). In flexion, mean valgus laxity was 2.5° (SD 1.5, range 0.0°–6.0°) and mean varus laxity was 3.1° (SD 2.0, range 0.1°–7.0°). Varus and valgus knee laxity in extension and in flexion were comparable. This study shows that the normal knee in this age group has an inherent degree of varus–valgus laxity. Whether the results of the present study can be used to optimise the total knee arthroplasty implantation technique requires further investigation. © 2007 Elsevier B.V. All rights reserved. Keywords: Varus laxity; Valgus laxity; Collateral ligaments; Normal knees; Radiographs
1. Introduction The main goals of a primary total knee arthroplasty are to relieve the patient's pain and to restore the knee joint kinematics and stability to a normal level. Correct positioning of the components and adequate soft-tissue balancing in extension and flexion are essential to achieve proper kinematics and stability [1,2]. Insall et al. (1976, 1985) described in his classic approach how to tension a knee in flexion using spreaders after ligamentous release in extension to align the leg [3,4]. However, the optimal amount of force on the ligament spreaders is unknown. Also unknown is how tight a knee should be when tested with spacers during implantation. A high force applied to the ligament capsule of the knee could result in a total ⁎ Corresponding author. Tel.: +31 24 3659628; fax: +31 24 3659154. E-mail address: [email protected] (P.J.C. Heesterbeek). 0968-0160/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2007.09.007
knee prosthesis being implanted “too tightly”. This might cause limited range of motion. On the other hand, a low amount of force applied to the ligaments could result in a knee with a high degree of instability. While post-operative range of motion might be greater it could be accompanied by instability problems in flexion [5] and may even lead to failure of the prosthesis [6]. In order to determine how tight the prosthetic knee should be balanced, it is important to know the laxity in a healthy, nonarthritic knee. Obviously this cannot be quantified following the procedure used during TKA implantation. Therefore, indirect methods such as stress X-rays are needed to quantify the laxity of healthy knees. In addition it is important to determine varus and valgus laxity separately, since the anatomy and, therefore, the kinematics of the medial and lateral side differ [7–9]. Stähelin et al. (2003) reported a radiographic technique to quantify in vivo varus and valgus knee joint laxity in flexion [10], but they did not report the values of laxity found in their study population.
46
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
Fig. 1. Flexilax: assessment of varus and valgus laxity of the knee in 70° flexion. A. Lateral view, B. view from above.
Within the diversity of reports available, most of which focused on varus–valgus laxity in extension or 20° of flexion, we found no quantitative data for varus–valgus laxity in flexion for the healthy older knee. The laxity in older subjects is of great clinical relevance because this would apply to most TKA patients. The objective of this study was to quantify the normal values for varus and valgus knee laxity in extension and flexion in healthy older subjects, thereby providing guidelines for the orthopaedic surgeon to restore the stability in the osteoarthritic knee and to optimise the total knee implantation techniques. 2. Methods 2.1. Subjects Before starting the study, a power calculation was carried out, and 28.7 patients were needed. The standard deviation and accuracy from a previous pilot study were 4.1° and 1.5°, respectively, with α set at 0.05 and β at 0.80. In total, 30 healthy subjects participated in this study (average age 62 (SD 6.4) years, 15 male, 15 female). The subjects were selected through an advertisement in the waiting room of our orthopaedic clinic and were included if their age was between 50–75, they had no osteo- or rheumatoid arthritis, no history of knee injury or knee complaints, and were able to walk for at least 1 h. Exclusion criteria were; previous hip arthrodesis or hip prosthesis, BMI N 35, knee flexion b 90°, treatment for osteoporosis, and readily visible varus or valgus leg alignment. For each subject one, randomly selected, knee was included in the study (15 left and 15 right knees). All subjects gave informed consent. The local ethics committee approved this study.
2.2. Measurements 2.2.1. Range of motion Passive range of motion of the knee (ROM) was measured with a goniometer by the research nurse. Active knee flexion was defined as the amount of maximum flexion that the subject could perform during stance without lifting the leg using the arms.
2.2.2. Varus–valgus laxity in extension Medial and lateral laxity of the knee in extension was assessed using the Telos device (Fa Telos, Medizinisch-Technische GmbH, Griesheim, Germany) with the subject lying in a supine position with leg muscles relaxed. The Telos device was applied with 15Nm load on the extended leg relative to the level of the joint line. While medial and lateral forces were applied, radiographs were obtained in the anteroposterior view. The direction of the X-rays was parallel to the tibia joint surface, centred on the middle of the femorotibial joint space. In addition, a radiograph without any medially or laterally applied force was made to determine the neutral position. 2.2.3. Varus–valgus laxity in flexion For medial and lateral flexion laxity, a custom made stress device was used to stress the knee and to produce reproducible measurements (Fig. 1). Before starting this study the reproducibility was thoroughly assessed. The 95% prediction limit (Bland and Altman) was 1.7° based on two radiographs of the same subject with a short interval between the radiographs and with the subject being repositioned (unpublished data). During the measurement the subject was in supine position with the lower leg on a plateau with the knee flexed in 70°. To minimize hip rotation the upper leg was strapped on a thigh support. Although measurement of flexion laxity is normally performed in 90° flexion, with this method radiographic examination in 90° knee flexion is not possible because the femur interferes with positioning of the radiographic equipment. The foot was held in an ankle–foot-orthosis. An external moment of 15 Nm was applied at the knee joint using 50 N on a pulley 0.30 m distal from the joint line (Fig. 1). The knee was stressed medially and laterally. Radiographs were made with the X-ray direction parallel to the tibia joint surface in the conditions varus, valgus or no moment applied. The subjects were instructed to relax their thigh muscles to minimize the role of the dynamic knee stabilizers. 2.2.4. Varus–valgus laxity calculation The angle between a tangent line on the femur condyles and a line through the deepest tibial joint surfaces was determined on the varus, valgus and neutral radiographs (Fig. 2) using the measurement tool within the radiographic database program (WEB1000 version 4.1, Agfa-Geveart AG, Rijswijk, The Netherlands). Valgus laxity was defined as the difference between the medial stress radiograph and the neutral radiograph, varus laxity as the difference between the lateral stress radiograph and the neutral radiograph. The measurements were made to the nearest 0.1°.
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
47
Fig. 2. Measurements of varus and valgus laxity on stress X-rays in flexion: neutral situation (A), valgus stress (B) and varus stress (C). 2.2.5. Statistical analyses Differences in varus and valgus laxity in extension and flexion for gender, were assessed using Student's t-test. The Shapiro–Wilk test was used to test for normal distribution. The association between age and varus–valgus laxity was investigated with a regression analysis as a regression of laxity on age with “age” as independent and “laxity” as dependent variable. Pearson's correlation coefficients (2-tailed) were calculated between varus and valgus laxity in extension, and varus and valgus laxity in flexion, and between varus laxity in extension and flexion and valgus laxity in extension and flexion. Results were presented as mean (SD) with range and 95% CI. All tests were performed with a level of significance of 0.05.
3.3. Varus–valgus laxity in flexion
Measurements could be performed on all subjects and no subject had to be excluded because of asymptomatic, radiographic arthritis.
The mean varus laxity in flexion was 3.1° (SD 2.0) and mean valgus laxity was 2.5° (SD 1.5) (Table 1). The Shapiro–Wilk test showed that both varus and valgus laxity in flexion were normally distributed (p = 0.1 and p = 0.6 for varus and valgus laxity, respectively). No difference in gender was found for varus laxity (p = 0.1) or valgus laxity (p = 0.2). There was a statistically significant association between age and varus laxity; varus laxity (degrees) = − 6.54 + 0.512⁎(degrees/years) (p = 0.004). There was no statistically significant association between age and valgus laxity. Pearson's correlation coefficient between varus and valgus laxity in flexion was 0.044 (p = 0.82). Between varus laxity in extension and flexion R = −0.043 (p = 0.82) and between valgus laxity in extension and flexion R = 0.41 (p = 0.025).
3.1. Range of motion
4. Discussion
Mean passive ROM was 132.3° (SD 7.3, range 115–140°). Mean active flexion was 115.7° (SD 15.2, range 80–140°). There were no significant correlations between either ROM or active flexion and varus– valgus laxity in extension or flexion.
The objective of this study was to quantify the amount of varus and valgus laxity in extension and flexion in the healthy older knee. There was a broad range for varus and valgus laxity, in extension as well as in flexion. This high inter-subject variability also seems to indicate that within individuals with functionally normal knees, the normal knee laxity can exist within a broad range. This study revealed a significant positive correlation between valgus laxity in extension and flexion, although only 17% of the amount of variability in valgus laxity in flexion could be explained by valgus laxity in extension. Overall the conclusion can be drawn that if a subject's knee is loose in extension, this does not mean that it also will be loose in flexion. The same applies for varus and valgus laxity. The data in the present study showed that higher age led to higher varus laxity in flexion. One other study also concluded that there was a (quite low; r = 0.29) correlation between varus–valgus laxity and age, as measured in extension [11]. In that study, older subjects had higher varus–valgus laxity than younger subjects. A point of discussion concerning the current study is the technical limitation imposed by not being able to measure in vivo varus–valgus laxity in 90° flexion with radiographs. Nevertheless, interpolation of data from Markolf et al. (1976) shows that the difference between laxity in 70° and 90° knee flexion with a
3. Results
3.2. Varus–valgus laxity in extension Mean varus laxity in extension was 2.8° (SD 1.3); mean valgus laxity in extension was 2.3° (SD 0.8) (Table 1). The Shapiro–Wilk test showed a normal distribution for both varus and valgus laxity (p = 0.8 and p = 0.4 for varus and valgus laxity, respectively). There was no significant gender difference for varus laxity (p = 0.2) or valgus laxity (p = 0.6). There was no statistically significant association between age and varus laxity or between age and valgus laxity. Pearson's correlation coefficient (R) between varus and valgus laxity in extension was −0.196 (p = 0.30).
Table 1 Descriptive data of varus and valgus laxity in extension and flexion
Extension varus laxity Extension valgus laxity Flexion varus laxity Flexion valgus laxity (n = 30).
Mean (°)
SD (°)
Range (°)
95% CI (°)
2.8 2.3 3.1 2.5
1.3 0.8 2.0 1.5
0.6–5.4 0.2–4.1 0.1–7.0 0.0–6.0
2.4–3.3 2.0–2.6 2.4–3.9 2.0–3.1
48
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
moment of 15 Nm is at the most 0.5° [12]. Although varus–valgus laxity should ideally be measured in 90° since the extension and flexion gap in a total knee prosthesis have an angle of 90°, we expect that our study will still be useful to provide guidelines for varus and valgus laxity in flexion and extension in TKA. Our in vivo findings approximate the in vitro knee laxities found during biomechanical tests of cadaver knee specimen by Markolf et al. [12] (Table 2). The cadaver study by Van Damme et al. (2005) found for extension similar laxity [13]. However, their results in flexion were higher than our in vivo results. A possible explanation for these differences might be the presence of muscular tension in our subjects that resulted in lower flexion laxity. Furthermore, their measurements were performed in mm, which when converted to degrees, could lower the laxity values. But they are still more than found in our study. Other in vivo knee laxity studies reported in the literature were performed with younger adults (Table 2). Okazaki et al. (2006) measured varus and valgus laxity on radiographs with knees stressed with Telos in 10° and 80° flexion in 19 to 59 years old individuals. Varus (lateral) laxity was significantly higher than valgus (medial) laxity in their study [14]. Possibly, the different measurement method may explain these differences. Another in vivo study in younger adults (aged 18 to 53) performed by Tokuhara et al. (2004) used MRI scans in 90° knee flexion with an unknown amount of load applied in 90° flexion. They also reported a higher mean varus (lateral) laxity than valgus (medial) laxity [15]. Both studies were performed in younger adults in a Japanese study population. Cultural differences, for example sitting position, and also anatomical differences between the Asian and Caucasian population might account for the higher lateral flexion laxity. Since the present study is the first to quantify varus and valgus laxity in extension and flexion in older, healthy (Caucasian) subjects, it remains difficult to compare the present results to the literature. In TKA the question is what amount of laxity is desired to obtain a good result. In the literature, studies report that a laxity of approximately 3° to 4° in extension for varus and valgus laxity is suitable to obtain good functional results in PCL retaining TKA [16–18]. Another study in bilateral TKA revealed that patients preferred the slacker knee above the tighter knee [19]. Drawing on the results of the present study, we would advise limiting the laxity in extension as well as flexion to less than 4°. Given the broad range of normal varus– valgus laxity it might be helpful to assess the varus–valgus Table 2 Literature overview Author
Markolf et al. [12] Van Damme et al. [13] Okazaki et al. [14] Tokuhara et al. [15]
In vivo/cadaver study
Extension laxity
Flexion laxity
Varus
Valgus
Varus
Valgus
Cadaver
1.5°
1.5°
5.5°
4.5°
Cadaver
3.1 mm (SD 0.8) 4.9° (SD 2.0) –
2.6 mm (SD 1.0) 2.4° (SD 1.6) –
8.1 mm (SD 1.0) 4.8° (SD 3.2) 6.7 mm (SD 1.9)
7.1 mm (SD 1.4) 1.7° (SD 1.4) 2.1 mm (SD 1.1)
In vivo, age 19–59 yr In vivo, age 18–53 yr
laxity in the healthy contralateral knee in case of TKP in a patient with unilateral osteoarthritis when deciding the correct ligament tension. Considering the advantages of our method to measure flexion laxity using radiographs, we can conclude that this method is a suitable repeatable method to evaluate flexion laxity in the natural knee. This flexion laxity measurement technique can also serve to analyse an unstable knee after total knee arthroplasty. This method enables a more precise measurement of varus and valgus laxity than can be obtained with physical examination. Clinical assessment of varus–valgus stability in the flexed knee is not always precise because tibiofemoral varus–valgus tilt can barely be distinguished from simultaneous hip rotation. Furthermore, the amount of joint opening is difficult to palpate and quantify, especially in heavier patients. Romero et al. investigated lateral flexion instability after TKA with an X-ray method which is comparable to the one used in the present study [20]. An interesting question is whether the presence of a certain amount of muscular tension may have influenced our results. Although the measurements were painless, the subjects did feel traction on their knee. These measurements were not performed in anaesthetized patients. During surgery there is certainly less muscular tension and knee laxity may be higher. However, the effect of muscular tension on knee laxity is unknown and should be investigated in future studies. The biomechanics of a replaced knee are different from those of a normal knee. It is therefore questionable if and how the normal values obtained in the present study can be applied to the total knee arthroplasty population. However, since this is still unknown, a good start may be to aim for a laxity that is within the normal range. With regard to the results reported and taking into consideration the repeatability errors that one can expect with this technique, we can provide the following recommendation to the surgeon: try to obtain a varus laxity in flexion between 0 and 7.1 degrees (95% confidence interval) and a valgus laxity between 0 and 5.5°. These values agree with the flexion laxity data of patients with good functioning total knee prostheses found by Romero et al. [20] In extension, the surgeon should try to obtain a varus laxity between 0.2 and 5.4° and a valgus laxity between 0.7 and 3.9°. Essentially, the results of this study indicate that the surgeon should balance the knee so that it does not have any varus–valgus laxity greater than 5.5 to 7°. Instead of using spacers or ‘feel’ to test varus–valgus laxity intra-operatively, the surgeon can use a navigation system in order to test varus–valgus laxity more accurately. When no navigation system is available, varus–valgus laxity can be estimated based on gap heights and goniometry. For example: for a varus laxity of 4°, the lateral gap has to be 3.5 mm greater than the medial gap when the distance between the contact points on the femoral condyles is 50 mm. From this study we can conclude that the range of varus– valgus laxity in extension and flexion in non-anaesthetised subjects without osteoarthritis is variable. Whether using these physiological laxity data for varus and valgus laxity during prosthetic implantation will affect clinical results in the long term is an important question and requires further investigation.
P.J.C. Heesterbeek et al. / The Knee 15 (2008) 45–49
Acknowledgements The authors like to thank research nurse Henriëtte de Gouw for her assistance and P.G. Anderson for her editorial support, and the RMS Foundation, Switzerland for financial support in this study. References [1] Fehring TK. Rotational malalignment of the femoral component in total knee arthroplasty. Clin Orthop 2000;380:72–9. [2] Whiteside LA. Soft tissue balancing: the knee. J Arthroplasty 2002;17:23–7. [3] Insall J, Ranawat CS, Scott WN, Walker P. Total condylar knee replacement: preliminary report. Clin Orthop Relat Res 1976;120:149–54. [4] Insall JN, Binazzi R, Soudry M, Mestriner LA. Total knee arthroplasty. Clin Orthop Relat Res 1985;192:13–22. [5] Zalzal P, Papini M, Petruccelli D, de BJ, Winemaker MJ. An in vivo biomechanical analysis of the soft-tissue envelope of osteoarthritic knees. J Arthroplasty 2004;19:217–23. [6] Fehring TK, Odum S, Griffin WL, Mason JB, Nadaud M. Early failures in total knee arthroplasty. Clin Orthop Relat Res 2001:315–8. [7] Hill PF, Vedi V, Williams A, Iwaki H, Pinskerova V, Freeman MA. Tibiofemoral movement 2: the loaded and unloaded living knee studied by MRI. J Bone Joint Surg Br 2000;82:1196–8. [8] Iwaki H, Pinskerova V, Freeman MA. Tibiofemoral movement 1: the shapes and relative movements of the femur and tibia in the unloaded cadaver knee. J Bone Joint Surg Br 2000;82:1189–95. [9] Nakagawa S, Kadoya Y, Todo S, Kobayashi A, Sakamoto H, Freeman MA, et al. Tibiofemoral movement 3: full flexion in the living knee studied by MRI. J Bone Joint Surg Br 2000;82:1199–200.
49
[10] Stähelin T, Kessler O, Pfirrmann C, Jacob HA, Romero J. Fluoroscopically assisted stress radiography for varus–valgus stability assessment in flexion after total knee arthroplasty. J Arthroplasty 2003;18:513–5. [11] Sharma L, Lou C, Felson DT, Dunlop DD, Kirwan-Mellis G, Hayes KW, et al. Laxity in healthy and osteoarthritic knees. Arthritis Rheum 1999;42: 861–70. [12] Markolf KL, Mensch JS, Amstutz HC. Stiffness and laxity of the knee— the contributions of the supporting structures. A quantitative in vitro study. J Bone Joint Surg Am 1976;58:583–94. [13] Van Damme G, Defoort K, Ducoulombier Y, Van Glabbeek F, Bellemans J, Victor J. What should the surgeon aim for when performing computerassisted total knee arthroplasty? J Bone Joint Surg Am 2005;87(Suppl 2): 52–8. [14] Okazaki K, Miura H, Matsuda S, Takeuchi N, Mawatari T, Hashizume M, et al. Asymmetry of mediolateral laxity of the normal knee. J Orthop Sci 2006;11:264–6. [15] Tokuhara Y, Kadoya Y, Nakagawa S, Kobayashi A, Takaoka K. The flexion gap in normal knees. An MRI study. J Bone Joint Surg Br 2004;86:1133–6. [16] 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. [17] Matsuda Y, Ishii Y, Noguchi H, Ishii R. Effect of flexion angle on coronal laxity in patients with mobile-bearing total knee arthroplasty prostheses. J Orthop Sci 2005;10:37–41. [18] Ishii Y, Noguchi H, Matsuda Y, Takeda M, Walker SA, Komistek RD. Effect of knee laxity on in vivo kinematics of meniscal-bearing knee prostheses. Knee 2007;14:269–74. [19] Kuster MS, Bitschnau B, Votruba 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] Romero J, Stahelin T, Binkert C, Pfirrmann C, Hodler J, Kessler O. The clinical consequences of flexion gap asymmetry in total knee arthroplasty. J Arthroplasty 2007;22:235–40.