Characteristics of femorotibial joint geometry in the trochlear dysplastic femur

Authors


Abstract

The medial and lateral tibia plateau geometry has been linked with the severity of trochlear dysplasia. The aim of the present study was to evaluate the tibial slope and the femoral posterior condylar offset in a cohort of consecutive subjects with a trochlear dysplastic femur to investigate whether the condylar offset correlates with, and thus potentially compensates for, tibial slope asymmetry. Magnetic resonance imaging was used to assess the severity of trochlear dysplasia as well as the tibial slope and posterior offset of the femoral condyles separately for the medial and lateral compartment of the knee joint in 98 subjects with a trochlear dysplastic femur and 88 control subjects. A significant positive correlation was found for the medial tibial slope and the medial posterior condylar offset in the study group (r2 = 0.1566; P < 0.001). This relationship was significant for all subtypes of trochlear dysplasia and was most pronounced in the severe trochlear dysplastic femur (Dejour type D) (r2 = 0.3734; P = 0.04). No correlation was found for the lateral condylar offset and the lateral tibial slope in the study group or for the condylar offset and the tibial slope on both sides in the control group. The positive correlation between the medial femoral condylar offset and the medial tibial slope, that is, a greater degree of the medial tibial slope indicated a larger offset of the medial femoral condyle, appears to represent a general anthropomorphic characteristic of distal femur geometry in patients with a trochlear dysplastic femur.

Introduction

The trochlear dysplastic femur plays a crucial in determining which patellae are likely to dislocate (Dejour et al. 1994). However, the geometry of the trochlear dysplastic femur is not only characterized by morphological changes to the trochlea but also to the posterior aspect of the distal femur (Van Haver et al. 2013). Recently, an increased femoral antetorsion (angle formed between the axis of the femoral neck and the posterior condylar axis of the distal femur) has been considered a primary risk factor for lateral patellar dislocation (LPD; Diederichs et al. 2013). Accordingly, further investigation of femoral malrotation and its origin has prompted a more detailed examination of the geometry of the tibial plateau in healthy subjects and in patients with patellofemoral instability. In particular, the inclination of the medial and lateral tibial plateau (slope) and its influence on femoral rotation during knee flexion has been described (Balcarek et al. 2013). In that study, the combination of a steep medial tibial slope with a flattened lateral tibial slope correlated with increasing severity of trochlear dysplasia (Balcarek et al. 2013). In addition, it was proposed that this tibial slope asymmetry might promote internal femoral rotation during knee flexion and thus potentially aggravate the effect of femoral antetorsion in the clinical setting of patellofemoral instability (Balcarek et al. 2013).

Corresponding to the posterior tibial slope (PTS) inclination, the femoral posterior condylar offset (PCO) might also influence femoral rotation during knee flexion and has been considered an important parameter of femoral component rotation in total knee arthroplasty (Bellemans et al. 2002). The PCO is defined as the shortest distance between a tangent along the dorsal femoral cortex and the most posterior portion of the condyles. In the literature, PCO measurements vary greatly in size, with some studies reporting a larger medial than lateral PCO in healthy people (Cinotti et al. 2012; Wang et al. 2014). Given the characteristics and the possible effects of tibial slope asymmetry on femoral rotation in patients with LPD, an isolated decrease in the medial PCO or an increase in the lateral PCO might also amplify internal femoral rotation during knee flexion. However, quantitative data regarding PCO characteristics in patients with LPD are not available, and no correlation between the PCO and the tibial slope asymmetry has been described in the literature. Therefore, the aim of this study was to evaluate the geometry of the PTS and PCO in a cohort of consecutive subjects with and without lateral patellar instability and to assess a possible correlation between them. The hypothesis of the present study was that the disparity of the PTS correlates with the size of the PCO in patients with LPD.

Materials and methods

Study group

Ninety-eight subjects (male/female 51/47; age range 11–48 years) with trochlear dysplasia were selected and included in the study group. All included patients were treated for LPD and underwent magnetic resonance imaging (MRI). The diagnosis of LPD was based on the medical history, a thorough clinical examination, and MRI investigation. Patients had to have a history of a primary or recurrent LPD including a documented dislocated patella that required reduction by an emergency doctor or a convincing history of giving way and clinical findings of joint effusion and tenderness along the medial patella facet, the medial retinaculum, or at the medial femoral condyle. Mechanisms of injury were either obtained from an accurately documented medical history or, in cases in which injury mechanisms were not recorded in detail, by retrospective personal questioning (Balcarek et al. 2010). The MRI criteria for LPD included joint effusion, contusion on the lateral femoral condyle or the medial patellar facet, osteochondral fragments, injury to the medial ligamentous stabilizers, the medial retinaculum and the medial patellofemoral ligament, and a lateralized patella. The MRI-based diagnosis of lateral patellar dislocation was made in patients who met three or more of the criteria as previously published (Balcarek et al. 2010). Exclusion criteria were fractures of the distal femur or tibial head, patients who underwent proximal or distal realignment procedures, and a multiligament injured knee.

Control group

Eighty-eight subjects (male/female 45/43; age range 10–57 years) without trochlear dysplasia were included in the control group. MRI diagnostics were performed during the same time period as the study group. The reasons for the MRI studies were internal derangements of the knee, including meniscus tear or cartilage lesions of the femorotibial joint. The exclusion criteria were a medical history of LPD; fractures of the distal femur or tibial head; and a tear of the anterior cruciate ligament because this injury is often associated with an increased inclination of the tibial slope in injuries sustained by non-contact mechanisms (Hashemi et al. 2010; Simon et al. 2010).

Image evaluation

Coronal, sagittal and transverse MRI images were available for all patients and were used both to assess the grade of trochlear dysplasia and to measure the medial and lateral tibial slope and the medial and lateral posterior femoral condylar offset. Measurements were performed using the annotation tools of a digital picture archiving and communication system (PACS) workstation (Centricity; GE Healthcare, St. Gilles, UK).

Trochlear dysplasia

Trochlear dysplasia was assessed on the transverse MR images, as described by Fucentese et al. (2006), and classified as type A–D according to the system described by Dejour et al. (1994) (Fig. 1A–D).

Figure 1.

(A) Type A: Trochlear morphology preserved with a fairly shallow trochlea; (B) Type B: Flat or convex trochlea; (C) Type C: Asymmetry of trochlear facets with convex lateral facet and hypoplastic medial facet; and (D) Type D: Asymmetry of trochlear facets, hypoplastic medial facet and cliff pattern.

Femoral posterior condylar offset (PCO)

The medial and lateral PCO were assessed according to the protocol proposed by Cinotti et al. (2012). The sagittal longitudinal axis of the femur was marked on the central sagittal MRI scan and was then transferred to the posterior femoral cortex (Fig. 2A). The centre of the lateral and medial condyles was identified on an axial scan (Fig. 2B). The tangent along the dorsal cortex was then transferred to the centre of the lateral and medial condyle in the sagittal plane (Fig. 2C). In this sagittal section, the shortest distance of each tangent along the posterior cortex (medial and lateral) to the most posterior extent of each condyle (double-headed arrow) was defined as the medial and lateral PCO.

Figure 2.

Assessment of the posterior condylar offset (PCO) on MRI. (A) Sagittal view; the longitudinal axis of the femur was marked (dotted line) and transferred to the dorsal cortex of the femoral shaft (solid line). (B) Axial view; marking the centre of the medial and lateral condyles (solid lines). (C) Sagittal scan; using a circle, the most posterior point of the condyles was identified. The shortest distance between the tangent along the posterior cortex of the femoral shaft and the most posterior point of the femoral condyle was defined as the PCO (double-headed arrow).

Posterior tibial slope (PTS)

The medial and lateral PTS were measured according to the protocol proposed by Hashemi et al. (2008). First, the longitudinal axis of the tibial diaphysis was identified. From the centre of the tibial head (solid line in Fig. 3), which was identified on the first proximal axial image of the tibial head, the corresponding sagittal slice was identified. In this sagittal slice, the line connecting the two points in the middle of the anterior-posterior extension of the tibial diaphysis (4–5 cm distal to the joint line and as distally as possible) was defined as the longitudinal axis of the tibia in the sagittal plane (Fig. 4A). The longitudinal axis was then transferred to the centre of the medial and lateral tibial head (shown as dashed lines in Fig. 3). In the corresponding sagittal slices, the reference for the tibial slope measurement was the line perpendicular to the longitudinal axis, starting at the intersection point of a marked line from the most anterior bony point to the most posterior bony point of the tibial plateau (posterior inclination), as determined separately for the medial and lateral tibial slope (Fig. 4B,C). The angle between the reference line and the posterior inclination of the tibia was defined as the PTS. The PTS was defined as positive when the posterior point of the tibial plateau was located under the reference line and was defined as negative if this point was located above this reference line.

Figure 3.

Axial scan of the entire tibial head. Determination of the central tibial head (solid line) and the centre of the medial and lateral tibial plateau (dashed lines).

Figure 4.

Sagittal scan of the proximal tibia. (A) The longitudinal axis of the tibial diaphysis was defined by the midpoint of the anterior-posterior distance at two locations (4–5 cm distal to the joint line and as distally as possible). The longitudinal axis was transferred to the centre of the medial plateau (B) and the lateral plateau (C). The line perpendicular to the longitudinal axis was used as the reference line for tibial slope measurements. The posterior tibial inclination was defined as the conjunction line between the peak anterior and peak posterior point of the tibial plateau. The angle between the reference line and the posterior inclination of the tibia was defined as the PTS.

Statistical analysis

All analyses were performed with the graphpad prism program (version 4; GraphPad Software, San Diego, CA, USA). Data were checked for Gaussian distribution and are presented as absolute frequencies, median, and range. Fisher's exact test was used to test for categorical values, and an unpaired two-tailed t-test was used to compare the medians. Linear regression analysis was used to determine the relationships between the PTS and the PCO, and was performed separately for the medial and lateral knee compartment. The influence of the grade of trochlear dysplasia on the medial and lateral PCO was also assessed by linear regression, with trochlear dysplasia used as an independent variable. In addition, a subgroup analysis evaluated the relationship between the PTS and PCO in skeletally mature and immature subjects stratified by age (< 18 years and > 18 years). Intra- and interrater reliability was assessed by two measurement series that were performed on 15 randomly selected subjects, either drawn repeatedly by one single rater over a 2-week interval or drawn independently by two different raters. Reliability was assessed by the correlation (Pearson's r) between the two measurement series or by the mean difference (t-test) between them.

Results

Patient demographics and the study parameters in the study and control groups are presented in Table 1. Although neither the median values of the medial and lateral PCO nor the medial to lateral PCO ratio differed between the study group and the control group, a significant positive correlation was identified for the medial PTS and medial PCO in trochlear dysplasia (r2 = 0.1566; P < 0.001) (Fig. 5A), i.e. a greater degree of the medial tibial slope indicated a larger offset of the medial femoral condyle. Although comparable, this relationship was slightly more pronounced in skeletally mature subjects (r2 = 0.1878, P = 0.0003) than in skeletally immature patients (r2 = 0.1445, P = 0.022). In addition, the relationship was significant for all subtypes (type A–D) of trochlear dysplasia and was most pronounced in the type D dysplasia group (r2 = 0.3734) (Fig. 6). No correlation was found for the lateral PCO and PTS in the study group (Fig. 5C) or for the medial and lateral PCO and PTS in the control group (Fig. 5B,D), irrespective of skeletal maturity. Additionally, the lateral tibial slope was slightly flatter in the study group than the control group (5° vs. 6.5°; P < 0.01).

Table 1. Demographics and parameter values of the study group and control group.
 Study group (n = 98)Control group (n = 88)P-value
  1. ns, non-significant; PCO, posterior condylar offset; PTS, posterior tibial slope.Values are presented as median and range.

Age, years25 (11–48)26 (10–57)ns
Male/female51/4745/43ns
Medial PCO, mm31.45 (21.9–42.1)31.30 (21.2–44.7)ns
Lateral PCO, mm31.10 (21.5–41.7)30.70 (14.3–40.3)ns
Ratio medial/lateral PCO1.01 (0.64–1.67)1.01 (0.61–1.85)ns
Medial PTS, degree7.0 (−1.5–20)8.0 (−1.0–14.5)ns
Lateral PTS, degree5.0 (−5.0–18.0)6.5 (0.0–15.5)< 0.01
Figure 5.

(A–D) Linear regression analysis evaluating the relationship between the posterior condylar offset of the distal femur and the tibial slope in the study group and control group as obtained separately for both the medial and lateral knee compartment.

Figure 6.

Linear regression analysis evaluating the relationship between the posterior condylar offset of the distal femur and the tibial slope in the medial knee compartment of the study group according to the different grades of trochlear dysplasia.

The inter- and intrarater reliability was highly positively significant for all investigated parameters, with a Pearson's r ranging of 0.77–0.89 (P = 0.006–0.0008). Concomitantly, the differences between the means were non-significant for all of the measurement series.

Discussion

Recently, an association between the tibial plateau geometry and the severity of trochlear dysplasia has been described in patients with lateral patellar instability (Balcarek et al. 2013). A medial-to-lateral tibial slope asymmetry was evident in severe trochlear dysplastic femora, thereby possibly aggravating the effect of femoral antetorsion during knee flexion. However, femoral rotation during knee flexion may also be influenced by the configuration of the distal femur, i.e. the radius of curvature of the femoral condyles or the posterior condylar offset. While Balcarek et al. (2013) did not observe a difference between the medial and lateral condylar radii, they did not investigate the PCO. Recently, however, the PCO has increasingly been viewed as being significant for knee joint movement patterns (Bellemans et al. 2002; Hoshino et al. 2012). Thus, the aim of the present study was to evaluate the geometry of the PTS and PCO in a cohort of consecutive subjects with trochlear dysplasia to examine whether the PCO correlates with, and thus potentially compensates for, tibial slope asymmetry in patients with LPD. The findings from our study indicate a positive correlation between the steepness of the medial PTS and the size of the medial PCO in the trochlear dysplasic femur. Thus, the positive correlation between the PCO and the PTS might compensate for the tibial slope asymmetry and appears to represent a general anthropomorphic characteristic of distal femur geometry associated with trochlear dysplasia and a steep medial tibial slope.

The adoption of bipedal locomotion in humans is associated with three distinct features of the lower limb. These features include the bicondylar angle of the femur, the prominence of the lateral trochlear lip and the elliptical profile of the lateral femoral condyle (Tardieu & Trinkaus, 1994; Tardieu et al. 2006). Although the increase in bicondylar angle is closely related to the ability to stand and walk in early childhood (Tardieu et al. 2006), it has been concluded that the geometry of the trochlear groove is already well developed at birth, favouring a genetic origin of trochlear groove anatomy (Glard et al. 2005). Recently, Van Haver et al. (2013) found not only that the trochlear region is affected in the trochlear dysplastic femur, but that the posterior femur also has a different morphology. In addition, the geometry of the tibial plateau has been linked to the severity of trochlear dysplasia (Balcarek et al. 2013), indicating a close relationship between the patellofemoral and femorotibial joints. In particular, the medial and lateral tibial slope has been linked to the influence of knee joint motion on the strain biomechanics and injury mechanisms of the cruciate ligaments and has been found to influence the peak stance knee internal rotations during a dynamic landing task (Amiri et al. 2007; Hashemi et al. 2010; McLean et al. 2010). Similarly, the influence of the PCO on the biomechanics of the knee joint and its implications for femoral component rotation in total knee arthroplasty have been reported recently (Bellemans et al. 2002; Hoshino et al. 2012). Moreover, modifications regarding the PCO in patients with LPD that might affect femoral rotation during knee flexion are also conceivable. Although the PCO and PTS seem to be closely related during knee joint flexion, most studies have assessed both parameters as separate entities.

Cinotti et al. (2012), who were the first to study the relationship between the PCO and the sagittal slope of the tibial plateau, observed a significant correlation between the PCO and the PTS in the medial knee compartment, although the same finding was not observed in the lateral compartment. Similarly, our results signify a positive correlation between the medial PCO and medial PTS in the study group (P < 0.001), which was significant for all subtypes (Type A–D) of trochlear dysplasia and was most pronounced in type D trochlear dysplastic femora. This finding is in agreement with a previous study in which the severity of trochlear dysplasia was significantly associated with an increase in tibial slope asymmetry. This finding may signify that the tibial slope asymmetry, as characterized by a steep medial slope and a flattened lateral slope in patients with LPD (Balcarek et al. 2013), may be compensated by a larger PCO medially. For unknown reasons that will require further study, we observed this correlation only in the study group and not in the controls. This finding differs from the results obtained by Cinotti et al. (2012) and remains debatable. Although it was not significant, a trend towards a positive correlation was also noted for our control group (P = 0.08).

The mean values of medial and lateral PCO measurements were 30.70–31.45 mm for both groups in our study. These data slightly exceeded the values of Cinotti et al. (2012) and those of Wang et al. (2014). The groups reported means of 27.4 and 25.2 mm and of 27.32 and 25.80 mm for the PCO measurements on the two sides, respectively. This difference may be attributable to different measurement techniques with reference to either bony or cartilaginous landmarks. In addition, a morphometric analysis revealed that the trochlear dysplastic femur is also characterized by 6–8% larger posterior condyles (lateral-medial) in the anteroposterior direction and a 6% larger medial condyle in the proximodistal direction compared with a normal femur (Van Haver et al. 2013).

In addition to patellar instability, trochlear morphology has been identified as a potential risk factor for patellofemoral osteoarthritis (Jungmann et al. 2013) that can be treated by patellofemoral arthroplasty in a middle-aged patient and by total knee arthroplasty (TKA) in the elderly (Ackroyd et al. 2007). Therefore, knowledge of the normal anatomic relationships between the distal femur and the geometry of the proximal tibia would appear mandatory for the replication of an individual condylar offset and tibial slope to obtain a balanced and almost parallel flexion gap in TKA. To the best of our knowledge, this study is the first to describe a characteristic variation of knee joint anatomy in patients with patellar instability that considers the relationship between the PCO and PTS in the medial knee compartment. However, several limitations were noted and deserve mention. First, access to a sufficient length of the femur and tibia on MRI and the ability to identify landmarks precisely may all have impacted the measurement accuracy. Secondly, the tibial slope measurements were determined by bony landmarks. However, the tibial head anatomy and its impact on knee joint biomechanics is also characterized by its cartilage surface, which is concave medially and convex laterally. Finally, the observers could not be blinded as to whether the images that were evaluated were obtained from the study group or the control group because several aspects of patellar instability are typically apparent on each image.

Conclusion

The findings of the present study introduce a positive correlation between the steepness of the tibial slope and the size of the posterior condylar offset in the medial compartment of the knee joint in association with a trochlear dysplastic femur. This correlation was significant for all subtypes of trochlear dysplasia and appears to represent a general anthropomorphic characteristic of distal femur geometry in patients with trochlear dysplasia.

Authors’ contributions

S.F.: drafting of the manuscript, data analysis. T.B. and J.P.S.: data acquisition. M.M.W. and T.A.W.: data analysis. K.M.S.: critical revision of the manuscript. P.B.: concept and design, data interpretation, approval of the article.

Conflict of interest

The authors report no potential conflict of interest.

Ancillary