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Patellar tilt: The physical examination correlates with MR imaging
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The Knee 15 (2008) 3 – 8
Patellar tilt: The physical examination correlates with MR imaging Ronald P. Grelsamer a,⁎, Craig H. Weinstein b , Jason Gould c , Ashok Dubey d a
Patellofemoral Reconstruction, Mount Sinai Medical Center, New York NY, USA b Sports & Orthopaedic Specialists, Mesa, AZ, USA c Mount Sinai Medical Center, New York NY 10029, USA d NYU/Hospital for Joint Diseases, New York NY 10003, USA
Received 7 May 2007; received in revised form 22 August 2007; accepted 31 August 2007
Abstract Patella malalignment is a recognized cause of knee pain, tilt being one of its more common forms. Although patellar tilt has been described both on the physical examination and on computerized imaging, to date the correlation between the two has not been established. A strong correlation would strengthen the value of each. Moreover, in situations where tilt cannot be clinically assessed (e.g. obesity), CT or MR imaging could be an adequate substitute for the clinical determination of tilt. We propose to correlate the physical examination with the magnetic resonance examination by way of an MR Tilt Angle. This angle is measured in a manner similar to the assessment of tilt on the physical examination, in that a line is drawn across the medial and lateral borders of the patella and referenced off the posterior femoral condyles. Most tilt angles use the slope of the lateral facet as a measure of tilt. These tilt angles paradoxically diminish as patellar tilt increases, a potential source of confusion. In this study, we use an MRI tilt angle that increases in the same direction as the actual tilt, which is more intuitive. We examined 30 patients with tilt and 51 patients without tilt. Patients with significant tilt on the physical examination can be expected to have an MRI Tilt Angle that is 10 degrees or greater whereas an angle of less than 10 degrees is associated with the absence of significant tilt on the physical examination. This MRI Tilt Angle fills the need for an easy, objective, intuitive measure of tilt and is an excellent adjunct to the physical examination. © 2007 Elsevier B.V. All rights reserved. Keywords: MRI; MRI Tilt Angle; Patellar tilt; Knee pain; Diagnostic study; Patellofemoral
1. Introduction Anterior knee pain can originate from a large number of conditions including overuse [1], bursitis, plicae, synovitis, muscle tightness, and patella malalignment [2–6]. When rotational malalignment (tilt) is noted on the physical examination and suspected to be the source of pain, objective documentation of this tilt is desirable. A number of X-ray, CT, and MRI parameters have been developed for this purpose (Table 1). Such documentation, however, is usually missing from magnetic resonance imaging (MRI) reports presumably from lack of a sufficient standard. Many tilt angles are descendants of the classic lateral patellofemoral angle (LPA) of Laurin [7], where decreasing degrees of radiographic tilt correlate with increasing degrees of ⁎ Corresponding author. Mount Sinai Medical Center, 5 East 98th Street, Box 1188, New York, NY 10029, USA. Tel.: +1 212 241 2914; fax: +1 212 534 6202. E-mail address: [email protected] (R.P. Grelsamer). 0968-0160/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2007.08.010
clinical tilt. Angles of 7 degrees (below which tilt is considered abnormal [8]) may not be practical or easy to remember. We sought to evaluate a tilt angle that reflects tilt on the physical examination, in the sense that increases in tilt would be reflected by higher values of the MRI tilt angle. The MRI tilt angle in this study is the one described in a small study by Powers [9] and similar to the one described by NoveJosserand and DeJour [10] for use on CT scans. The tilt angle is subtended by the plane of the posterior femoral condyles and a line drawn connecting the medial and lateral borders of the patella (Fig. 1). Use of this line is consistent with the determination of tilt on the physical examination [11] and has been previously described in the evaluation of patellar tilt on plain radiographs [12], Computerized Tomography (CT) scans [10,13–16] and MR imaging [9,17,18]. Use of the posterior femoral condyles has also been previously utilized by a number of investigators [10,19–25]. We sought to answer two related questions: (1) Does the MRI tilt angle differ significantly between a group of patients with tilt on
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Table 1 Review of the literature Author
Year
Number of Controls/Study patients
Country
Imaging modality
Method (⁎)
Laurin [7] Delgado-Martins [14] Boven [45] Martinez [21] Sasaki [15] Schutzer [25] Fernandez de Rota [46] Inoue [47] Kujala [18] Shellock [48] Reikeras [49] Brossman [19] Grelsamer [12] Koskinen [17] Pinar [23,24] Nove-Josserand [10] Delgado-Martinez [20] Vahasarja [16] Dupuy [50] Powers [9] Muhle [51] Delgado-Martinez [13] McNally [52] Present study
1978 1980 1982 1983 1986 1986 1988 1988 1989 1989 1990 1993 1993 1993 1994 1995 1996 1996 1997 1998 1999 2000 2000 2003
100/30 12/0 0/71 10/5 12/20 10/24 0/49 30/50 20/0 14/130 43/0 15/13 100/100 15/11 14/26 27/110 0/18 0/31 0/18 12/3 Not Applicable (Review) 0/14 0/474 51/30
Canada Switzerland Belgium USA Japan USA Spain Japan Finland USA Norway Germany USA Finland Turkey France Spain Finland USA USA Germany Spain UK USA
X-ray CT CT CT CT CT CT CT MRI MR CT MRI X-ray MRI CT CT CT CT CT MRI CT / MRI CT MRI MRI
Laurin(a) Delgado-Martins(b) Laurin Martinez(c) Delgado-Martins Martinez Laurin Laurin Delgado-Martins Not specified Laurin Martinez Grelsamer(d) Delgado-Martins/ Laurin Martinez Nove-Josserand(e) Martinez Delgado-Martins/ Laurin Laurin Nove-Josserand Martinez Delgado-Martins/ Martinez Laurin Nove-Josserand
Notes: (⁎) Description of method: angle line drawn through patella/reference line on femur. (a) Martinez: Lateral facet/posterior condyles. (b) Delgado-Martins: Medial-lateral borders/anterior condyles. (c) Laurin: Lateral facet/anterior condyles. (d) Grelsamer: Medial-lateral borders/horizontal plane. (e) Nove-Josserand: Medial-lateral borders/posterior condyles.
the physical examination when compared with an unmatched group of patients without such tilt? and (2) can the MRI tilt angle be used to successfully predict the presence or absence of patellar tilt? 2. Materials and method
consecutive patients with knee pain who exhibited irreducible patellar tilt (tilt that could not be passively corrected to neutral). The patients were not selected for any particular disease, symptom or age. Inclusion criteria for our study included a recording in the chart of the presence or absence of irreducible tilt on the physical examination and the existence of an MRI of that same knee.
Patellar tilt as was evaluated in two consecutive series of patients with knee pain: Group I (Controls) consisted of 51 patients with knee pain without evidence of patellar tilt on the physical examination. Group II [26] consisted of 30
Fig. 1. The MRI tilt angle is subtended by the plane of the posterior femoral condyles and a line drawn connecting the medial and lateral borders of the patella. The angle is assessed on the first cut that images the medial and lateral borders of the patella as well as the posterior femoral condyles.
Fig. 2. Clinical assessment of patellar tilt. An imaginary line drawn from the medial to the lateral border of the patella should be horizontal. Alternatively, a tilted patella (lateral side down) should be easily reducible to a neutral position.
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Fig. 3. Distribution of MRI tilt angles among Control patients. Tilt on the physical examination was assessed by palpating the medial and lateral borders of the patella with the knee extended or slightly flexed, as described by Kolowich [11] (Fig. 2). The normal patella lies in the frontal plane. If it is tilted, the lateral border of the patella lies in a plane posterior to that of the medial border. Normally, the lateral border can be passively elevated until the patella is “horizontal”, or “un-tilted”. If a tilted patella cannot be so manipulated, the lateral retinaculum is tight and the patient is said to exhibit patellar tilt. It is important to note that patellar tilt may or may not be responsible for knee symptoms in any given patient. The evaluation of tilt was part of a comprehensive patellar examination performed on any patient with knee pain. Excluded were patients whose patella could not be palpated because of obesity. Also excluded were patients who had undergone prior knee surgery including arthroscopy. No patient had a greater than 2 over 4 effusion (where 1 would be the mildest and 4 the most severe effusion). The physical examination was carried out and recorded prior to the evaluation of any additional diagnostic studies. The senior investigator performed all examinations. All MRIs were carried out within our geographic region where MRI knee coils impose approximately 10 degrees of knee flexion [27]. The subjects' quadriceps were relaxed. Going from a proximal to distal direction we measured tilt on the first cut that images the medial and lateral borders of the patella as well as the posterior femoral condyles and the inter-condylar notch (Fig. 1).
2.1. Statistical methods Continuous data measurements were screened for normalcy prior to application of parametric statistics. Between-group comparisons were made using the Students' t-test for unpaired comparisons with an alpha threshold of 0.05. With the numbers available in this study, there is an 88% statistical power
Fig. 5. Abnormal patellar tilt (N10 degrees). to detect a difference of 5 degrees between positive and negative MRI tilt values (actual difference 12.2 degrees). Prior to further analysis, the results of this MRI measurement were analyzed graphically (Fig. 3) to determine a suitable threshold level for distinguishing between positive and negative values. By applying the predetermined cutoff value, a prediction of the expected patellar tilt was made. By comparing this to the clinical examination, diagnostic indices (i.e. sensitivity and specificity) for each measurement could be determined. In comparing these levels of observation, Chi-square analysis was employed with p-values derived by use of Fisher's exact test where appropriate.
3. Results In Group I (Controls), 88% (45/51) had an MRI tilt angle ≤ 10°, 96% (49/51) had an MRI tilt angle ≤15° (Fig. 3). In Group II [26] 100% (30/30) of patients had a tilt angle ≥10° (Figs. 4 and 5). In comparing these two groups, we found a mean MRI tilt angle of 6 ± 5.5 degrees among the Control group and 18 ± 7 degrees among the Study group (p b 0.0001). In judging the ability of the MRI tilt angle to predict tilt on the physical examination, we found a sensitivity of 87%, a specificity of 88%, a positive predictive value of 81%, and a negative predictive value of 92% when using 10 degrees as the upper limit of normal. The tilt angle was in agreement with the clinical determination of patellar tilt in 88% (45/51) of patients in the Control group and in 87% (26/30) of patients in the Study group. Overall, there was agreement between the radiographic and clinical examination in 88% (71/81) of cases. Ten degrees of tilt seems to be a dividing line, as we found a cluster of patients around that mark (10 in the Control group and 4 in the tilt group). Because there were many more controls than study patients at the 10 degree mark, we chose to make 10 degrees the upper limit of normal.
4. Discussion
Fig. 4. Distribution of MRI tilt angles among Study patients.
The sources of patellar pain are varied, excessive lateral tilting of the patella being just one of many, and the subject matter continues to be a source of interest to the orthopedic community [28–30] (as are all manners of MRI analyses of the patellofemoral joint) [31–34]. This study should not be interpreted to mean that the detection of abnormal patellar tilt in any given patient automatically implies
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that malalignment is the source of that particular patient's pain. The clinician must make that determination on a case by case basis. The history, physical examination, and radiographs remain the most important tools at the clinician's disposal, the MRI being often unnecessary. Once the MRI has been ordered, however, one cannot overstate the importance of noting the presence or absence of tilt. Such documentation provides objective confirmation of the clinician's findings and can draw the clinician's attention to a finding he or she might have overlooked on the physical examination. The purpose of this study has been to determine the extent to which an MRI correlates with the physical examination. Determining radiographic tilt requires a tilt line and a reference line, with the angle subtended by these two lines representing the tilt angle. Laurin was one of the first to establish such an angle (8). As his tilt line he chose the plane of the lateral (bony) facet of the patella. This approach was carried over to CT scanning by Martinez [21] and then by Schutzer et al. [25]. This approach leads to a tilt angle that varies in the reverse direction of the clinical tilt: Tilt angles diminish as tilt on the physical examination increases and vice versa. Since MRI scans are particularly suited to visualizing articular cartilage, one could also consider a tilt line drawn along one of the cartilaginous surfaces of the patella if one postulates that the position of the cartilage is more important than that of the bone [17]. This argument is bolstered by the patellar cartilage's particularity of not following the contour of the subchondral bone [35,36]. However, since the purpose of this study is to correlate the MRI tilt angle with findings on the clinical examination it would appear logical to judge tilt in a similar fashion. On the physical examination tilt is assessed by palpating the medial and lateral borders of the patella [11,37]. As far back as 1941 Wiberg noted a variety of patellar shapes when the patellofemoral joint is viewed in the axial plane [38]. Some patellae feature a relatively steep lateral facet where in others it is practically horizontal. A tilt angle that makes use of the lateral facet will vary with the morphology of the patella even in the absence of any change in true tilt. For example, in a patella shaped like the so-called Hunter's cap the Laurin tilt angle will be markedly abnormal even if the patella is normally positioned. Images of the patellofemoral joint obtained with a subject in the supine position and the quadriceps relaxed may or may not present a true reflection of the patella's position during every day activities [10,15,20,39]. The same applies to the physical examination. The purpose of this study, however, is to correlate the MRI findings with the physical examination. Most studies on patellar tilt have utilized either very few subjects or very few controls. Sixteen of the twenty-two papers reviewed examined fewer than 20 controls, and ten papers enrolled fewer than 20 abnormals (see Table 1). Our study includes 30 study patients and 51 controls. The inter-observer and intra-observer variability of radiographic tilt angles has been studied and found to be acceptable, most likely because of the relative ease of identifying the landmarks involved [13]. If anything, the reliability in our study is enhanced by the fact that we need not determine the specific value of the angle, only whether an angle is greater or lesser
than 10 degrees. We did not assess the inter-and intra-observer variability of the clinical assessment of patellar tilt, and this could be a potential future study. Imperfect inter-and intraobserver correlations might explain the lack of a perfect correlation between the clinical and the MRI evaluation of tilt. Whether the physical examination or the MRI evaluation of tilt should be considered the reference against which the other should be measured is subject to debate. The MRI evaluation is objective, demonstrates good inter-observer and intra-observer reliability, and therefore could be considered more reliable than the physical examination. However, the distal femur can exhibit both internal and external torsion. As such, use of the posterior condyles as a reference plane can artificially affect the measurement of tilt (Fig. 6). In a number of our patients with a “false positive” tilt angle (tilt N 10° on the MRI but normal tilt on the physical examination), the tilt was the result of internal rotation of the distal femur. Thus, physiologically, these false positives may in fact be true positives. In our study, the same investigator assessed patellar tilt on the physical examination and on the MRIs, a potential source of bias. The presence of major outliers in the control group, however, suggests that bias was not a significant factor. Nevertheless, our results and conclusions might be considered preliminary pending further investigation. The study by Nove-Josserand and Dejour [10] and the study by Powers et al. [9] used a tilt angle similar to ours. NoveJosserand and Dejour published a CT study involving a total of 110 patients, including 27 controls. Their study focused on instability, i.e. abnormal, symptomatic lateral displacement of the patella. They noted a threshold of 20 degrees above which subjects were likely to suffer from instability. They also noted parenthetically that tilt was significantly increased in their abnormal sub-groups when the quadriceps were contracted, confirming the prior work of Sasaki [15], Delgado-Martinez [20], and Guzzanti [36]. Powers et al. [9] utilized the same MRI tilt angle that we have used in this study and applied it to a small group of controls (12) and a smaller number of patients with patellofemoral symptoms (3). All of their control patients
Fig. 6. Abnormal tilt angle produced by internal rotation of the femur.
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demonstrated tilt angles of less than 10 degrees, which is consistent with our results. Tracking studies in which the position of the patella is assessed during cycling of the knee require precise and reproducible determinations of knee flexion [40,41]. This is also true if one wishes to compare tilt before and after the institution of a treatment protocol. The exact degree of knee flexion, however, is not critical when determining whether tilt on routine MRI imaging correlates with the physical examination, as evidenced by the strong correlation obtained in this study. Such was already the case in our prior study of patellar translation on MR imaging [42]. A knee effusion can affect patellar tracking [43] and if severe enough could in theory increase or diminish patellar tilt. No patient in this study had more than a mild effusion. We found a cluster of patients around the 10 degrees mark. Because there were many more controls than study patients at that mark, we chose to make 10 degrees the upper limit of normal. Had we instead chosen 10 degrees to be the lower limit of abnormal our sensitivity and negative predictive value would have risen to 100% at the expense of our specificity and positive predictive value (69% and 65% respectively). Phrased differently, if a clinician were to consider 10 degrees (and all angles greater) to be abnormal on an MRI he would not miss a single patient with abnormal tilt on the physical examination. Interpreted in such a manner this tilt angle would be the perfect screening test for abnormal tilt. However, the clinician would be over-reading tilt in a significant number of patients. In everyday practice an MRI tilt of 10 degrees is simply a borderline reading that doesn't strongly suggest the presence or absence of clinical tilt. In summary, though doubt has been cast on the reliability of patellar tilt measurements for predicting the presence or absence of tilt on the physical examination [44], it appears that this MRI Tilt Angle does indeed accurately reflect such tilt. Patients with tilt on the physical examination can be expected to have an MRI Tilt Angle that is 10 degrees or greater, while an MRI tilt angle of less than 10 degrees is strongly associated with an absence of clinical tilt. The MRI Tilt Angle fills the need for a simple, intuitive, practical, and valid measure of patellar tilt. References [1] Dye SF. The knee as a biologic transmission with an envelope of function: a theory. Clin Orthop 1996;325:10–8. [2] Ficat P, Ficat C, Bailleux A. External hypertension syndrome of the patella. Its significance in the recognition of arthrosis. Rev Chir Orthop Repar Appar Mot 1975;61(1):39–59. [3] Ficat P, Philippe J, Cuzacq JP, Cabrol S, Belossi J. The syndrome of external hyperpressure of the patella. A radioclinical entity. J Radiol Electrol Med Nucl Dec 1972;53(12):845–9. [4] Fulkerson JP. Diagnosis and treatment of patients with patellofemoral pain. Am J Sports Med May–Jun 2002;30(3):447–56. [5] Grelsamer RP. Patellar malalignment. J Bone Jt Surg Am Nov 2000;82-A (11):1639–50. [6] Merchant AC, Mercer RL, Jacobsen RH, Cool CR. Roentgenographic analysis of patellofemoral congruence. J Bone Jt Surg Am 1974;56 (7):1391–6. [7] Laurin CA, Levesque HP, Dussault R, Labelle H, Peides JP. The abnormal lateral patellofemoral angle: a diagnostic roentgenographic sign of recurrent patellar subluxation. J Bone Jt Surg Am 1978;60(1):55–60.
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The Knee 15 (2008) 3 – 8
Patellar tilt: The physical examination correlates with MR imaging Ronald P. Grelsamer a,⁎, Craig H. Weinstein b , Jason Gould c , Ashok Dubey d a
Patellofemoral Reconstruction, Mount Sinai Medical Center, New York NY, USA b Sports & Orthopaedic Specialists, Mesa, AZ, USA c Mount Sinai Medical Center, New York NY 10029, USA d NYU/Hospital for Joint Diseases, New York NY 10003, USA
Received 7 May 2007; received in revised form 22 August 2007; accepted 31 August 2007
Abstract Patella malalignment is a recognized cause of knee pain, tilt being one of its more common forms. Although patellar tilt has been described both on the physical examination and on computerized imaging, to date the correlation between the two has not been established. A strong correlation would strengthen the value of each. Moreover, in situations where tilt cannot be clinically assessed (e.g. obesity), CT or MR imaging could be an adequate substitute for the clinical determination of tilt. We propose to correlate the physical examination with the magnetic resonance examination by way of an MR Tilt Angle. This angle is measured in a manner similar to the assessment of tilt on the physical examination, in that a line is drawn across the medial and lateral borders of the patella and referenced off the posterior femoral condyles. Most tilt angles use the slope of the lateral facet as a measure of tilt. These tilt angles paradoxically diminish as patellar tilt increases, a potential source of confusion. In this study, we use an MRI tilt angle that increases in the same direction as the actual tilt, which is more intuitive. We examined 30 patients with tilt and 51 patients without tilt. Patients with significant tilt on the physical examination can be expected to have an MRI Tilt Angle that is 10 degrees or greater whereas an angle of less than 10 degrees is associated with the absence of significant tilt on the physical examination. This MRI Tilt Angle fills the need for an easy, objective, intuitive measure of tilt and is an excellent adjunct to the physical examination. © 2007 Elsevier B.V. All rights reserved. Keywords: MRI; MRI Tilt Angle; Patellar tilt; Knee pain; Diagnostic study; Patellofemoral
1. Introduction Anterior knee pain can originate from a large number of conditions including overuse [1], bursitis, plicae, synovitis, muscle tightness, and patella malalignment [2–6]. When rotational malalignment (tilt) is noted on the physical examination and suspected to be the source of pain, objective documentation of this tilt is desirable. A number of X-ray, CT, and MRI parameters have been developed for this purpose (Table 1). Such documentation, however, is usually missing from magnetic resonance imaging (MRI) reports presumably from lack of a sufficient standard. Many tilt angles are descendants of the classic lateral patellofemoral angle (LPA) of Laurin [7], where decreasing degrees of radiographic tilt correlate with increasing degrees of ⁎ Corresponding author. Mount Sinai Medical Center, 5 East 98th Street, Box 1188, New York, NY 10029, USA. Tel.: +1 212 241 2914; fax: +1 212 534 6202. E-mail address: [email protected] (R.P. Grelsamer). 0968-0160/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2007.08.010
clinical tilt. Angles of 7 degrees (below which tilt is considered abnormal [8]) may not be practical or easy to remember. We sought to evaluate a tilt angle that reflects tilt on the physical examination, in the sense that increases in tilt would be reflected by higher values of the MRI tilt angle. The MRI tilt angle in this study is the one described in a small study by Powers [9] and similar to the one described by NoveJosserand and DeJour [10] for use on CT scans. The tilt angle is subtended by the plane of the posterior femoral condyles and a line drawn connecting the medial and lateral borders of the patella (Fig. 1). Use of this line is consistent with the determination of tilt on the physical examination [11] and has been previously described in the evaluation of patellar tilt on plain radiographs [12], Computerized Tomography (CT) scans [10,13–16] and MR imaging [9,17,18]. Use of the posterior femoral condyles has also been previously utilized by a number of investigators [10,19–25]. We sought to answer two related questions: (1) Does the MRI tilt angle differ significantly between a group of patients with tilt on
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Table 1 Review of the literature Author
Year
Number of Controls/Study patients
Country
Imaging modality
Method (⁎)
Laurin [7] Delgado-Martins [14] Boven [45] Martinez [21] Sasaki [15] Schutzer [25] Fernandez de Rota [46] Inoue [47] Kujala [18] Shellock [48] Reikeras [49] Brossman [19] Grelsamer [12] Koskinen [17] Pinar [23,24] Nove-Josserand [10] Delgado-Martinez [20] Vahasarja [16] Dupuy [50] Powers [9] Muhle [51] Delgado-Martinez [13] McNally [52] Present study
1978 1980 1982 1983 1986 1986 1988 1988 1989 1989 1990 1993 1993 1993 1994 1995 1996 1996 1997 1998 1999 2000 2000 2003
100/30 12/0 0/71 10/5 12/20 10/24 0/49 30/50 20/0 14/130 43/0 15/13 100/100 15/11 14/26 27/110 0/18 0/31 0/18 12/3 Not Applicable (Review) 0/14 0/474 51/30
Canada Switzerland Belgium USA Japan USA Spain Japan Finland USA Norway Germany USA Finland Turkey France Spain Finland USA USA Germany Spain UK USA
X-ray CT CT CT CT CT CT CT MRI MR CT MRI X-ray MRI CT CT CT CT CT MRI CT / MRI CT MRI MRI
Laurin(a) Delgado-Martins(b) Laurin Martinez(c) Delgado-Martins Martinez Laurin Laurin Delgado-Martins Not specified Laurin Martinez Grelsamer(d) Delgado-Martins/ Laurin Martinez Nove-Josserand(e) Martinez Delgado-Martins/ Laurin Laurin Nove-Josserand Martinez Delgado-Martins/ Martinez Laurin Nove-Josserand
Notes: (⁎) Description of method: angle line drawn through patella/reference line on femur. (a) Martinez: Lateral facet/posterior condyles. (b) Delgado-Martins: Medial-lateral borders/anterior condyles. (c) Laurin: Lateral facet/anterior condyles. (d) Grelsamer: Medial-lateral borders/horizontal plane. (e) Nove-Josserand: Medial-lateral borders/posterior condyles.
the physical examination when compared with an unmatched group of patients without such tilt? and (2) can the MRI tilt angle be used to successfully predict the presence or absence of patellar tilt? 2. Materials and method
consecutive patients with knee pain who exhibited irreducible patellar tilt (tilt that could not be passively corrected to neutral). The patients were not selected for any particular disease, symptom or age. Inclusion criteria for our study included a recording in the chart of the presence or absence of irreducible tilt on the physical examination and the existence of an MRI of that same knee.
Patellar tilt as was evaluated in two consecutive series of patients with knee pain: Group I (Controls) consisted of 51 patients with knee pain without evidence of patellar tilt on the physical examination. Group II [26] consisted of 30
Fig. 1. The MRI tilt angle is subtended by the plane of the posterior femoral condyles and a line drawn connecting the medial and lateral borders of the patella. The angle is assessed on the first cut that images the medial and lateral borders of the patella as well as the posterior femoral condyles.
Fig. 2. Clinical assessment of patellar tilt. An imaginary line drawn from the medial to the lateral border of the patella should be horizontal. Alternatively, a tilted patella (lateral side down) should be easily reducible to a neutral position.
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Fig. 3. Distribution of MRI tilt angles among Control patients. Tilt on the physical examination was assessed by palpating the medial and lateral borders of the patella with the knee extended or slightly flexed, as described by Kolowich [11] (Fig. 2). The normal patella lies in the frontal plane. If it is tilted, the lateral border of the patella lies in a plane posterior to that of the medial border. Normally, the lateral border can be passively elevated until the patella is “horizontal”, or “un-tilted”. If a tilted patella cannot be so manipulated, the lateral retinaculum is tight and the patient is said to exhibit patellar tilt. It is important to note that patellar tilt may or may not be responsible for knee symptoms in any given patient. The evaluation of tilt was part of a comprehensive patellar examination performed on any patient with knee pain. Excluded were patients whose patella could not be palpated because of obesity. Also excluded were patients who had undergone prior knee surgery including arthroscopy. No patient had a greater than 2 over 4 effusion (where 1 would be the mildest and 4 the most severe effusion). The physical examination was carried out and recorded prior to the evaluation of any additional diagnostic studies. The senior investigator performed all examinations. All MRIs were carried out within our geographic region where MRI knee coils impose approximately 10 degrees of knee flexion [27]. The subjects' quadriceps were relaxed. Going from a proximal to distal direction we measured tilt on the first cut that images the medial and lateral borders of the patella as well as the posterior femoral condyles and the inter-condylar notch (Fig. 1).
2.1. Statistical methods Continuous data measurements were screened for normalcy prior to application of parametric statistics. Between-group comparisons were made using the Students' t-test for unpaired comparisons with an alpha threshold of 0.05. With the numbers available in this study, there is an 88% statistical power
Fig. 5. Abnormal patellar tilt (N10 degrees). to detect a difference of 5 degrees between positive and negative MRI tilt values (actual difference 12.2 degrees). Prior to further analysis, the results of this MRI measurement were analyzed graphically (Fig. 3) to determine a suitable threshold level for distinguishing between positive and negative values. By applying the predetermined cutoff value, a prediction of the expected patellar tilt was made. By comparing this to the clinical examination, diagnostic indices (i.e. sensitivity and specificity) for each measurement could be determined. In comparing these levels of observation, Chi-square analysis was employed with p-values derived by use of Fisher's exact test where appropriate.
3. Results In Group I (Controls), 88% (45/51) had an MRI tilt angle ≤ 10°, 96% (49/51) had an MRI tilt angle ≤15° (Fig. 3). In Group II [26] 100% (30/30) of patients had a tilt angle ≥10° (Figs. 4 and 5). In comparing these two groups, we found a mean MRI tilt angle of 6 ± 5.5 degrees among the Control group and 18 ± 7 degrees among the Study group (p b 0.0001). In judging the ability of the MRI tilt angle to predict tilt on the physical examination, we found a sensitivity of 87%, a specificity of 88%, a positive predictive value of 81%, and a negative predictive value of 92% when using 10 degrees as the upper limit of normal. The tilt angle was in agreement with the clinical determination of patellar tilt in 88% (45/51) of patients in the Control group and in 87% (26/30) of patients in the Study group. Overall, there was agreement between the radiographic and clinical examination in 88% (71/81) of cases. Ten degrees of tilt seems to be a dividing line, as we found a cluster of patients around that mark (10 in the Control group and 4 in the tilt group). Because there were many more controls than study patients at the 10 degree mark, we chose to make 10 degrees the upper limit of normal.
4. Discussion
Fig. 4. Distribution of MRI tilt angles among Study patients.
The sources of patellar pain are varied, excessive lateral tilting of the patella being just one of many, and the subject matter continues to be a source of interest to the orthopedic community [28–30] (as are all manners of MRI analyses of the patellofemoral joint) [31–34]. This study should not be interpreted to mean that the detection of abnormal patellar tilt in any given patient automatically implies
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that malalignment is the source of that particular patient's pain. The clinician must make that determination on a case by case basis. The history, physical examination, and radiographs remain the most important tools at the clinician's disposal, the MRI being often unnecessary. Once the MRI has been ordered, however, one cannot overstate the importance of noting the presence or absence of tilt. Such documentation provides objective confirmation of the clinician's findings and can draw the clinician's attention to a finding he or she might have overlooked on the physical examination. The purpose of this study has been to determine the extent to which an MRI correlates with the physical examination. Determining radiographic tilt requires a tilt line and a reference line, with the angle subtended by these two lines representing the tilt angle. Laurin was one of the first to establish such an angle (8). As his tilt line he chose the plane of the lateral (bony) facet of the patella. This approach was carried over to CT scanning by Martinez [21] and then by Schutzer et al. [25]. This approach leads to a tilt angle that varies in the reverse direction of the clinical tilt: Tilt angles diminish as tilt on the physical examination increases and vice versa. Since MRI scans are particularly suited to visualizing articular cartilage, one could also consider a tilt line drawn along one of the cartilaginous surfaces of the patella if one postulates that the position of the cartilage is more important than that of the bone [17]. This argument is bolstered by the patellar cartilage's particularity of not following the contour of the subchondral bone [35,36]. However, since the purpose of this study is to correlate the MRI tilt angle with findings on the clinical examination it would appear logical to judge tilt in a similar fashion. On the physical examination tilt is assessed by palpating the medial and lateral borders of the patella [11,37]. As far back as 1941 Wiberg noted a variety of patellar shapes when the patellofemoral joint is viewed in the axial plane [38]. Some patellae feature a relatively steep lateral facet where in others it is practically horizontal. A tilt angle that makes use of the lateral facet will vary with the morphology of the patella even in the absence of any change in true tilt. For example, in a patella shaped like the so-called Hunter's cap the Laurin tilt angle will be markedly abnormal even if the patella is normally positioned. Images of the patellofemoral joint obtained with a subject in the supine position and the quadriceps relaxed may or may not present a true reflection of the patella's position during every day activities [10,15,20,39]. The same applies to the physical examination. The purpose of this study, however, is to correlate the MRI findings with the physical examination. Most studies on patellar tilt have utilized either very few subjects or very few controls. Sixteen of the twenty-two papers reviewed examined fewer than 20 controls, and ten papers enrolled fewer than 20 abnormals (see Table 1). Our study includes 30 study patients and 51 controls. The inter-observer and intra-observer variability of radiographic tilt angles has been studied and found to be acceptable, most likely because of the relative ease of identifying the landmarks involved [13]. If anything, the reliability in our study is enhanced by the fact that we need not determine the specific value of the angle, only whether an angle is greater or lesser
than 10 degrees. We did not assess the inter-and intra-observer variability of the clinical assessment of patellar tilt, and this could be a potential future study. Imperfect inter-and intraobserver correlations might explain the lack of a perfect correlation between the clinical and the MRI evaluation of tilt. Whether the physical examination or the MRI evaluation of tilt should be considered the reference against which the other should be measured is subject to debate. The MRI evaluation is objective, demonstrates good inter-observer and intra-observer reliability, and therefore could be considered more reliable than the physical examination. However, the distal femur can exhibit both internal and external torsion. As such, use of the posterior condyles as a reference plane can artificially affect the measurement of tilt (Fig. 6). In a number of our patients with a “false positive” tilt angle (tilt N 10° on the MRI but normal tilt on the physical examination), the tilt was the result of internal rotation of the distal femur. Thus, physiologically, these false positives may in fact be true positives. In our study, the same investigator assessed patellar tilt on the physical examination and on the MRIs, a potential source of bias. The presence of major outliers in the control group, however, suggests that bias was not a significant factor. Nevertheless, our results and conclusions might be considered preliminary pending further investigation. The study by Nove-Josserand and Dejour [10] and the study by Powers et al. [9] used a tilt angle similar to ours. NoveJosserand and Dejour published a CT study involving a total of 110 patients, including 27 controls. Their study focused on instability, i.e. abnormal, symptomatic lateral displacement of the patella. They noted a threshold of 20 degrees above which subjects were likely to suffer from instability. They also noted parenthetically that tilt was significantly increased in their abnormal sub-groups when the quadriceps were contracted, confirming the prior work of Sasaki [15], Delgado-Martinez [20], and Guzzanti [36]. Powers et al. [9] utilized the same MRI tilt angle that we have used in this study and applied it to a small group of controls (12) and a smaller number of patients with patellofemoral symptoms (3). All of their control patients
Fig. 6. Abnormal tilt angle produced by internal rotation of the femur.
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demonstrated tilt angles of less than 10 degrees, which is consistent with our results. Tracking studies in which the position of the patella is assessed during cycling of the knee require precise and reproducible determinations of knee flexion [40,41]. This is also true if one wishes to compare tilt before and after the institution of a treatment protocol. The exact degree of knee flexion, however, is not critical when determining whether tilt on routine MRI imaging correlates with the physical examination, as evidenced by the strong correlation obtained in this study. Such was already the case in our prior study of patellar translation on MR imaging [42]. A knee effusion can affect patellar tracking [43] and if severe enough could in theory increase or diminish patellar tilt. No patient in this study had more than a mild effusion. We found a cluster of patients around the 10 degrees mark. Because there were many more controls than study patients at that mark, we chose to make 10 degrees the upper limit of normal. Had we instead chosen 10 degrees to be the lower limit of abnormal our sensitivity and negative predictive value would have risen to 100% at the expense of our specificity and positive predictive value (69% and 65% respectively). Phrased differently, if a clinician were to consider 10 degrees (and all angles greater) to be abnormal on an MRI he would not miss a single patient with abnormal tilt on the physical examination. Interpreted in such a manner this tilt angle would be the perfect screening test for abnormal tilt. However, the clinician would be over-reading tilt in a significant number of patients. In everyday practice an MRI tilt of 10 degrees is simply a borderline reading that doesn't strongly suggest the presence or absence of clinical tilt. In summary, though doubt has been cast on the reliability of patellar tilt measurements for predicting the presence or absence of tilt on the physical examination [44], it appears that this MRI Tilt Angle does indeed accurately reflect such tilt. Patients with tilt on the physical examination can be expected to have an MRI Tilt Angle that is 10 degrees or greater, while an MRI tilt angle of less than 10 degrees is strongly associated with an absence of clinical tilt. The MRI Tilt Angle fills the need for a simple, intuitive, practical, and valid measure of patellar tilt. References [1] Dye SF. The knee as a biologic transmission with an envelope of function: a theory. Clin Orthop 1996;325:10–8. [2] Ficat P, Ficat C, Bailleux A. External hypertension syndrome of the patella. Its significance in the recognition of arthrosis. Rev Chir Orthop Repar Appar Mot 1975;61(1):39–59. [3] Ficat P, Philippe J, Cuzacq JP, Cabrol S, Belossi J. The syndrome of external hyperpressure of the patella. A radioclinical entity. J Radiol Electrol Med Nucl Dec 1972;53(12):845–9. [4] Fulkerson JP. Diagnosis and treatment of patients with patellofemoral pain. Am J Sports Med May–Jun 2002;30(3):447–56. [5] Grelsamer RP. Patellar malalignment. J Bone Jt Surg Am Nov 2000;82-A (11):1639–50. [6] Merchant AC, Mercer RL, Jacobsen RH, Cool CR. Roentgenographic analysis of patellofemoral congruence. J Bone Jt Surg Am 1974;56 (7):1391–6. [7] Laurin CA, Levesque HP, Dussault R, Labelle H, Peides JP. The abnormal lateral patellofemoral angle: a diagnostic roentgenographic sign of recurrent patellar subluxation. J Bone Jt Surg Am 1978;60(1):55–60.
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