The most important finding of this study is that the classification described—which includes new, more accurate criteria for assessing TD—may provide relatively reliable guidance for trochleoplasty performed to treat TD. We obtained the most accurate true lateral-view photographs using engineering software. Even minimal rotation aberrations of 5° during conventional radiography causes false-positive or false-negative results . This novel classification may reduce the false-positive and false-negative rates, especially when it is difficult to obtain TLCRs.
Previous studies have found that various selected levels for magnetic resonance imaging (MRI) reflect different TD types [5, 21]. The Dejour classification of TD shows low agreement between conventional radiography and axial MRI [5, 14]. Sabine et al. showed that intra-observer and inter-observer agreement is only fair with Dejour’s four-type classification (24–78%) . Gian et al. showed that plain axial conventional radiography does not represent the true bony trochlear morphology and may result in inappropriate clinical management . Philippe et al. believed that axial MRI analysis of the entire distal femur can obtain more accurate clinical classification . Because of the influence of selected levels of CT/MRI [5, 22], our study mainly focused on the true lateral view of the TD. Our research aimed to reevaluate the true lateral view of femoral models in patients with TD using 3D software. The false-positive and false-negative rates can be minimized by using the most accurate true lateral view imaging.
Trochleoplasty is designed to change the morphology of the trochlea to stabilize an unstable patella [9, 23, 24]. Patellofemoral congruence may be promoted by lateral-facet elevating trochleoplasty or by sulcus-deepening trochleoplasty . Albee’s lateral-facet elevating trochleoplasty has been nearly abandoned because of the great pressure it applies on the lateral patellofemoral area and the subsequent patellofemoral osteoarthritis . Sulcus-deepening trochleoplasty , first described by Masse in 1978, has been the basis for several techniques and modified techniques [23, 24, 27–30]. The sulcus-deepening trochleoplasty is designed to eliminate the prominence of the femoral trochlea and re-shape a groove of suitable depth [9, 23].
Among the four types of TD in our 96 cases, type 1 was found in 7.3%, type 2 in 19.8%, type 3 in 13.5%, and type 4 in 59.4%. Type 1 TD deserves little consideration of trochleoplasty. Other surgical procedures (e.g., medial patellofemoral ligament reconstruction, tibial tuberosity medialization osteotomy) should be considered for the management of patellofemoral instability. Type 2 TD is characterized by a bump of the lateral condyle or a supratrochlear spur that often causes great pressure on the lateral patellofemoral joint. Arthroscopic lateral patelloplasty  or arthroscopic deepening trochleoplasty  may be considered if the patient has lateral patellar compression syndrome. These techniques can reduce pressure on the articular surface of the lateral patellofemoral area, thereby relieving symptoms. We suggest that both type 3 and type 4 require sulcus-deepening trochleoplasty. The crossing point located in the distal area of the trochlea means that the trochlea is fairly shallow over a long distance. Longo et al. compared the clinical outcomes of these patients treated with three trochleoplasty procedures . Dejour’s “V-shaped” sulcus deepening trochleoplasty, Bereiter’s “U-shaped” deepening trochleoplasty, and Goutallier’s “recession trochleoplasty” are associated with significantly improved stability and function and a low rate of osteoarthritis and pain. However, the sulcus-deepening trochleoplasty is never performed alone as other procedures must be combined with it to correct patellofemoral instability .
The “crossing sign” of Dejour’s classification is a distinguishing feature of TD . Dejour et al. noted that the crossing sign is different in three types of TD .
Type I: The crossing of the two condylar outlines with the trochlear groove line is symmetrical and proximal.
Type II: The two condyles are asymmetrical, and the trochlear groove line crosses first the medial condyle, at a variable level, and then the lateral condyle.
Type III: The crossing of the two condylar outlines with the trochlear groove line is symmetrical and situated distally.
It should be noted that we did not observe that the trochlear groove line crossed with the medial condyle first and then crossed the lateral condyle. We only found two types of crossing sign. The most common was that the trochlear groove line crossed the medial condyle but did not cross the lateral condyle (68.9%). Another form was defined as the trochlear groove line crossing the two condylar outlines at the same point (31.1%). We believe that the site of the crossing point is more important than the form of the crossing point.
Dejour et al. described a cortical beak (later called the supratrochlear spur), which is the superolateral trochlear facet (type III) . Later, in 2006, Dejour et al. described type D, which had a convex lateral facet . We think that the two qualitative criteria are controlled by the same mechanism but in different trochlear positions (Fig. 4). There is no consensus on whether the lateral condyle bump should be removed surgically. In our opinion, trochleoplasty (perhaps in concert with other procedures) is required if the enlarged lateral facet or the supratrochlear spur affects patellar tracking or increases pressure on the patellofemoral joint. When the patella is taken through its whole range of motion, there must be no patellar impingement during the process .
According to Dejour’s classification, the double contour is a marked characteristic of types C and D [9, 13]. The reason we do not use the double contour as a reference for classification is that the double contour does not represent features of the trochlear facets. As far as we know, the double contour is not caused by projection of the medial facet of the trochlea. It is caused by the extended anterior cortex of the bilateral trochlea—there is no cartilage. Although the incidence of the double-contour sign in all cases was 62.5%, rotation of 5°–10° caused the double contour sign to disappear in 56.7% (34/60) of cases. Therefore, the rotation that occurs while obtaining lateral conventional radiographs may lead to misclassification.
This study has several limitations. First, although four types of TD were found in this study, there may be other types. A larger sample may be needed. Although this classification may not be perfect, it may indicate that the appearance of TD is diverse. Second, the 1-mm CT slices may have resulted in the loss of some information. Perhaps 0.625-mm CT slices would have been a better choice. Third, these images generated by the engineering software may not be exactly the same as the true lateral view of conventional radiographs. Nevertheless, automated classification might be a good option in the future .