The patellofemoral (PF) joint has a low degree of congruency by nature; therefore, it is more susceptible to lateral dislocation.

A normal trochlea is located in the anterior aspect of the distal femur. It consists of two facets divided by a longitudinal groove (the trochlear sulcus). The lateral facet is the largest, extending more proximally than the medial one and is more protuberant in the anteroposterior aspect.

The proximal border of the trochlea is represented by the junction of the articular cartilage with the femoral anterior cortex, which is covered by adipose tissue and synovium.

Distally, the trochlear surface is separated from its corresponding condylar surface on both sides by a slight groove (the condylotrochlear groove).

The trochlear sulcus extends distally up to the notch deviating slight laterally from the femoral axis.¹ Iranpour et al found that the line along the deepest points of the trochlear groove was aligned 1° ± 5° in valgus relative to the transcondylar axis corresponding more or less to the mechanical axis of the lower limb.²

Trochlear dysplasia refers to a pathologic alteration in the shape of the femoral trochlea. It is defined by a sulcus angle of more than 145°.³,⁴ Dysplastic trochleae are shallow, flat, or even convex and are not effective in constraining mediolateral patellar displacement.

Dejour et al⁵ identified three different populations with PF disorders:

Dejour et al⁵,⁶ established a classification of lateral patellar instability based on radiographic imaging and described four major anatomic factors for instability with statistical threshold. These four factors were trochlear dysplasia, patella alta (Caton-Deschamps index > 1.2), excessive distance between tibial tubercle (TT) and trochlear groove (TG) (TT-TG > 20 mm), and excessive lateral patellar tilt (>20°).

Trochlear dysplasia has been the main anatomic determinant; it has been present in 96% of the OPI population.

Trochlear dysplasia is a geometric deformity, especially at its proximal part, where the patella initially engages into the trochlea. It may lead to PF maltracking, increased contact pressures, patellar instability, and isolated PF arthritis.

In a cadaveric study using different types of trochlear custom-made implants, Van Haver et al⁷ investigated the effect of trochlea dysplasia on the PF biomechanics. There was an increase in patellar internal rotation, lateral tilt, lateral translation, and PF contact pressures and a decrease in PF contact area and stability in the trochlear dysplastic group compared with the control group.

The trochlear implants graded as Dejour type D (see section “Classification”) showed the largest deviations for the kinematic parameters, and the implants graded as Dejour types B and D revealed the largest deviations for the PF contact areas and pressures.⁷

The supratrochlear spur of the trochlea increases the lateral patellar tilt and the PF compression forces when the knee is flexed.⁸,⁹

There is a high correlation between lateral patellar tilt and the type of trochlear dysplasia¹⁰; the higher the degree of trochlear dysplasia, the greater the degrees of lateral patellar tilt.

Trochlear dysplasia is classified by the use of x-rays and slice imaging (computed tomography [CT] or magnetic resonance imaging [MRI]).

On a lateral x-ray, with the knee flexed at 20° and with perfect superimposition of both femoral condyles, the normal trochlea is defined by the anterior contour of the facets/condyles and, posterior to them, by the line representing the deepest points (floor) of the trochlear sulcus.¹¹,¹²

The dysplasia is defined on the true lateral x-ray by the “crossing sign,”⁶ when the line representing the deepest part of the trochlear groove crosses or reaches the anterior border of the two condyles. The crossing point is the position where the floor of the trochlear sulcus reaches the height of the femoral condyles, meaning that the trochlea becomes flat in this exact location. In normal knees, this line representing the floor of the trochlear sulcus is usually 0.8 mm posterior to a line tangent to the anterior femoral cortex; in case of trochlea dysplasia, it is located about 3.2 mm anterior to this tangent.⁵

Two additional radiographic signs are described on the lateral view:

Based on these signs, Dejour¹³,¹⁴,¹⁵,¹⁶ classified trochlear dysplasia into the following four types (Figure 18.1):

Axial views have been described with different orientation of x-ray beam angles and knee flexion angles. Ideal axial view should be performed at lower knee flexion angles (30°), as the most proximal part of the trochlea, where dysplasia is common, could be better evaluated on these views.¹⁷ When images are performed at lower knee flexion angles, the lateral facet would represent two-third of the total trochlear width. Axial radiographs obtained with the knee flexed more than 45° are less useful as they show the distal part of the trochlea which is normal/deeper even in severe dysplasia and the patella is normally engaged in higher degrees of knee flexion.

Dysplastic trochleae would show higher (flatter) sulcus angles, some of which cannot be measured as there is no true sulcus.

Moreover, on the axial view, it is possible to evaluate for the presence of an avulsion fracture from the medial patella, articular congruence, cartilage thickness, and the presence of PF osteoarthritis.

The CT scan would allow better visualization of the trochlea shape in the axial plane and the assessment of associated features (patellar tilt, TT-TG distance, femoral anteversion, and external tibial torsion).

MRI is another modality to evaluate the anatomy of the cartilaginous layer of the trochlea. MRI axial images can distinguish between a low- and high-grade dysplasia with better interobserver agreement than lateral radiographs.¹⁸

Staubli et al¹⁹ observed that the cartilaginous shape of the trochlear sulcus does not necessarily follow the underlying bony anatomy. However, in flat or convex trochlea, the bony and cartilaginous surfaces seem to match more frequently (flat or convex bone will be covered by flat or convex cartilage).

Table 18.1 lists indications and contraindications for trochleoplasty.

Instability and pain are the two main presenting symptoms.

A true episode of patellar dislocation (with obvious deformity or hemarthrosis), mechanism of injury (low- or high-energy trauma), and whether the episode was painful or not should be documented.

The number of instability episodes and the dislocation rate should be noted (episodic, habitual, or permanent).

The mechanism of patellar reduction must be assessed, as to whether the patient could painlessly reduce the dislocation or if it required forceful reduction with or without sedation/anesthesia.

Physician must focus on knee morphology, patellar kinematics, and signs of patellar laxity.

Patient evaluation should start by noticing how the patient rises from a chair and walks into the clinic. This information is interesting as the patient would be acting naturally.

The patient should be evaluated in standing and siting position for any asymmetry at the level of the shoulders or pelvis.

Coronal and rotational plane alignment is observed for the presence of femoral anteversion, genu valgum, tibial torsion, and limb-length discrepancy.

Patellar position should be evaluated. Normally, the patellae should face forward. If they face inward, femoral anteversion may be present.

The overall extensor mechanism alignment is checked.

With the patient in standing position, subtalar eversion should be assessed from the posterior aspect because it can produce compensatory internal tibial rotation.

Gait evaluation is important because the rotational deformities are exacerbated and limping can become evident.

Patellar tracking is evaluated during active and passive knee movements. The J-sign is indicative of abnormal patellar tracking with lateral patellar displacement in full knee extension as soon as trochlear restraints fail to contain the patella. It is present in high-grade trochlear dysplasia.

Palpation should start in the less painful areas. The patella, patellar tendon, quadriceps tendon, lateral and medial retinacula, and patellar facets should be palpated for tenderness. It should be noted that retinacula are interposed between the examiner’s fingers and the patellar facets and may cause confusion as to the source of pain.

Glide test: Patellar medial and lateral glide should be tested at full extension and at 30° of knee flexion. Patella is divided into four vertical quadrants. Displacement of less than 1 quadrant or more than 3 quadrants is considered abnormal.

Lateral patellar tilt test is performed in full knee extension: The examiner tries to reduce lateral patellar tilt by
elevating its lateral facet and correcting the tilt. Inability to elevate the lateral facet is related to lateral retinacular tightness. Significant tilt on physical examination would correlate with CT/MRI tilt values.⁸

Smillie test²⁰ or apprehension test: It is performed with the knee extended. The examiner holds the patella with his fingers and applies a laterally directed force, trying to dislocate it while holding the tibia with the other hand. This laterally directed force applied over the patella can cause apprehension in the patient when they feel that the patella is about to dislocate. It is also called the apprehension test because it is the patient’s positive reaction that will determine test positivity. For adequate examination, the quadriceps should be relaxed. The test should be performed bilaterally because comparison with the opposite side can help. It is less useful in acute dislocation setting because pain, apprehension, or fear could be present even before the physical examination.

Hamstring tightness is tested with the patient supine and the opposite leg extended and flat over the table. The hip is flexed 90°, and the knee is then extended as far as possible. The popliteal angle is observed and compared with the opposite side.

Quadriceps tightness is tested with the patient prone, and in most patients, the heels should touch the buttocks.

The iliotibial band is tested with the Ober test: With the patient in lateral decubitus position (the side to be tested is up), the hip is extended and abducted, and the knee is extended. From this position, the thigh is released and is allowed to adduct. Most patients will be able to touch the examination table with the medial aspect of the knee.

Standard x-rays are of great value in PF disorders. The type of trochlear dysplasia is evaluated on true lateral x-rays in a standing position with 20° of knee flexion and with perfect superimposition of the posterior femoral condyles. It is advisable for the radiographs to be done under direct fluoroscopy and under physician supervision. Even minor degrees of knee rotation can underestimate the presence of trochlear dysplasia signs. In the same lateral views, measurement of patellar height according to Caton-Deschamps index²¹ must be performed.

Axial view in 30° of knee flexion would help to measure the sulcus angle and the lateral tilt of the patella.

Slice imaging with CT or MRI is mandatory to confirm the type of dysplasia (axial views), to measure the TT-TG distance, and to measure the lateral patellar tilt (with and without quadriceps contraction).

Sagittal MRI views are useful to evaluate any trochlear prominence and to measure the sagittal PF engagement between the trochlea and the patella.²²

The goal of the sulcus-deepening trochleoplasty is to undermine the subchondral bone of the trochlea through an osteocartilaginous osteotomy and to remove enough cancellous bone posterior to the trochlea so that the supratrochlear spur would disappear, without damaging the chondral surface.

The amount of bone removed should be enough to have a new trochlear groove that is flush with the anterior femoral cortex without any prominence. This procedure would lead to a more anatomic and concave trochlea by deepening the sulcus proximally. It would improve the patellar engagement in early degree of knee flexion.

Trochleoplasty is always combined with other softtissue procedures (eg, medial patellofemoral ligament [MPFL] reconstruction) and, possibly, with bony procedures (eg, tibial tuberosity osteotomy) in order to correct all the predisposing anatomic factors according to the “menu à la carte” introduced by Dejour.⁵

If the TT-TG distance is increased (>20 mm), a tibial tuberosity medialization is performed in order to obtain a postoperative TT-TG value between 10 and 15 mm.⁵,¹³ If the TT-TG distance is between 20 and 25 mm, a combined proximal realignment procedure can be performed by lateralizing the “new trochlear groove.” The trochleoplasty procedure can reduce the TT-TG distance by 5 and 10 mm.²³,²⁴

In the case of patella alta (Caton-Deschamps index > 1.2), distalization of the tibial tubercle is done in order to achieve a normal patellar index of 1.0⁵,¹³ and to improve the sagittal engagement index.²²

A lateral release/lengthening is performed in patients with a nonreducible lateral patellar tilt less than 20°.

The procedure is performed under general or regional anesthesia. The patient is positioned supine. The entire extremity is prepared and draped (Video 18.1 ).

The surgery technique follows 10 consecutive steps (Table 18.2).

The first step consists of trochlear exposure through a mid-vastus or a trans-quadriceps approach. With the knee flexed to 80°, a straight midline skin incision is made from the superior patellar margin to the
tibiofemoral joint line. Sharp dissection of the medial retinaculum is performed about 1- to 2-cm from the medial border of the patella, and blunt dissection of vastus medialis obliquus fibers is performed at the superomedial pole of the patella, extending proximally approximately 4 cm into the muscle belly (if mid-vastus approach) (Figure 18.2).

The patellar chondral surface is evaluated, and International Cartilage Repair Society classification is used to describe chondral lesions. Specific treatment (flap resection, microfracture, and autologous chondrocyte implantation) is performed if necessary. The patella is then retracted laterally but not everted.

The second step is represented by the trochlear assessment and the proximal realignment planning. The native trochlear groove is marked with a sterile pen. Next, two additional divergent lines starting at the notch and directed proximally through the condylotrochlear grooves (sulcus terminalis), representing the lateral and medial facet limits, are marked. They should not enter the tibiofemoral articulation. Finally, the new trochlear groove is marked in a more lateral position according to the preoperative TT-TG value. The proximal-distal limit of trochleoplasty is defined by the osteochondral edge proximally and the intercondylar notch distally (Figure 18.3).

The third step is the supratrochlear spur resection and access to trochlear undersurface. The peritrochlear synovium and periosteum are incised sharply along their osteochondral junction and reflected using a periosteal elevator. The anterior femoral cortex must be visible because it is used as a reference to determine the amount of cancellous bone that has to be removed. A strip of the proximal cortical bone, including the supratrochlear spur, is resected using a sharp osteotome. The amount of the strip is equal to the distance between the supratrochlear spur and the anterior femoral cortex. At the end of this step, all the cortical bone around the superior border of the trochlea is successfully detached (Figure 18.4).