Physeal-Sparing Medial Patellofemoral Ligament Reconstruction



Physeal-Sparing Medial Patellofemoral Ligament Reconstruction


Shital N. Parikh

Sean Keyes



INTRODUCTION



  • This chapter describes the surgical procedure for medial patellofemoral ligament (MPFL) reconstruction in skeletally immature patients using physeal-sparing femoral tunnel placement and pull-through of hamstring graft through the femoral tunnel (pull-through technique).


  • The importance of distal femoral physis and its relationship to the MPFL femoral attachment point is emphasized.


Pathogenesis



  • Fithian et al defined the highest risk population for patellar dislocation as females aged 10 to 17 years.1


  • Historically, the rate of patellar redislocation or persistent symptoms after first-time dislocation has been reported in the range of 17% to 44%.1,2,3,4


  • Lately, attempts have been made to predict recurrence after first-time dislocation to help in optimal management of patients. These prediction models are based on assessment of demographic and anatomic risk factors.5,6


  • The identified risk factors for recurrent instability after first-time patellar dislocation include age less than 14 years, skeletal immaturity, sports-related patellar dislocation, history of contralateral patellar dislocation, patella alta, patellar tilt, trochlear dysplasia, increased tibial tubercle-trochlear groove (TT-TG) distance, and hyperlaxity7,8,9 (Table 27.1).


  • Lewallen et al reported that skeletally immature patients with trochlear dysplasia had a 69% rate of recurrent instability after first-time dislocation.8


Applied Anatomy


Patellar Height



  • Ossification of the patella begins between age 3 and 5 years. This ossification may appear irregular but rapidly expands in a proximal to distal fashion. The ossification process is completed by the age of 16 to 17 years. Because of the delayed ossification of the distal aspect of the patella, patellar height calculations on radiographs may be spuriously elevated in children10 (Figure 27.1).


  • Caton-Deschamps index has been evaluated in children and has been found to be a simple and reliable measurement for patellar height, though age-based values have to be considered. The mean age-based values are 1.27 ± 0.25 (6-7 years), 1.15 ± 0.17 (8-9 years), 1.11 ± 0.19 (10-11 years), and 1.07 ± 0.18 by 12 years.11 A value greater than 1.45 (2 standard deviations above normal) was considered to be a prognostic risk factor for recurrent patellar dislocation.7


TT-TG Distance



  • A lateralized TT has been recognized as an important factor for patellar instability. The lateralization is measured by the TT-TG distance on axial MRI or CT study.








    TABLE 27.1 Predictors of Recurrent Dislocation After First-Time Patellar Dislocation

































    Risk Factors

    No. of Risk Factors

    Predicted Risk of Recurrence (%)

    Suggested Treatment




    • Skeletal immaturity

    0

    14

    Conservative




    • History of contralateral dislocation

    1

    30

    Conservative




    • Trochlear dysplasia

    2

    54

    Equivocal




    • Patella alta

    3

    75

    Can consider surgical stabilization

    4

    88

    Can consider surgical stabilization








    Figure 27.1 A, Lateral knee radiograph of a prepubescent boy shows the Caton-Deschamps index to be 27.5/16.2 = 1.69, which would indicate significant patella alta. B, Corresponding MRI sequence shows the unossified cartilage of patella and the proximal tibia and normal patellar height if these cartilage surfaces are considered.


  • The average TT-TG distance in adults with and without patellar instability has been reported as 19.8 ± 1.6 mm and 12.7 ± 3.4 mm, respectively.9 Values more than 20 mm have been used as an indication for TT medializing osteotomy, with a goal to restore the TT-TG distance between 10 and 15 mm.


  • As TT-TG is an absolute number, it does not account for the size of the knee or age of the patient. Studies have been done to develop TT-TG ratio or index to overcome this drawback.


  • TT-TG distance in children shows logarithmic progression with age, with values approaching adult values as children approach skeletal maturity. In normal children (without patellar instability), the mean TT-TG distance has been reported as 8.6 ± 0.3 mm at an average age of 11 years and 11 months. In children with patellar instability, the mean TT-TG distance has been reported as 12.2 ± 1.1 mm at an average age 13 years and 5 months. Thus, younger the patient, less is the threshold value for correction of TT-TG distance.12


Trochlear Dysplasia and Growth



  • On a lateral radiographic view, one of the three signs for trochlear dysplasia (crossing sign, supratrochlear bump, or double contour sign) was present in all skeletally immature patients with trochlear dysplasia.13


  • During growth, all linear measurements of trochlear dysplasia (condylar height, trochlear height, trochlear bump) increased with age. However, the shape of the TG (sulcus angle) did not change much with increasing age.14 Progressive ossification of trochlea may give a radiographic appearance of deepening of TG (decrease in sulcus angle) with skeletal growth; this is spurious finding.15


  • The TG can remodel following patellar stabilization, though the remodeling capacity decreases with age. After 10 years of age, there is no significant remodeling of the trochlea.16 Thus, if possible, earlier patellar stabilization should be offered to children under the age of 10 years.


  • Trochlear dysplasia is one of the main risk factors for patellar instability and failure of patellar stabilization surgery. In a group of 37 children with open physis, with mean age of 14 years and failed patellar stabilization surgery (lateral release, medial reefing, Roux-Goldthwait procedure), the authors found high-grade trochlear dysplasia in 89% of patients. Patella alta and increased TT-TG distance were not significant risk factors for failure.17


Anterior Distal Femoral Physis



  • The spatial and causal relationship between the anterior aspect of distal femoral physis and supratrochlear bump is less understood. The mean distance between the anterior aspect of distal femoral physis and the proximal-most aspect of the trochlea is around 4.5 mm. However, the lateral aspect of the trochlea can be proximal to the physis (13.7% knees) or at the level of the physis (17.7% knees) (Figure 27.2).






Figure 27.2 MRI sagittal sequence through lateral aspect of knee shows the trochlear articular cartilage (dashed arrow) extending proximal to the anterior distal femoral physis (solid arrow).







Figure 27.3 Anteroposterior view of left knee at 4 years, 8 years, 12 years, and 15 years. There are changes in the shape of the distal femoral physis with skeletal growth. The central ridge (black arrows) decreases in height relative to the metaphyseal-epiphyseal junction (white arrows). There is progressive cupping of the epiphysis such that the metaphyseal-epiphyseal junction is higher than the central ridge at skeletal maturity.

The distance between the trochlea and physis increases with skeletal growth, that is, the physis moves away from the proximal trochlea.18 This relationship is important to surgeons planning trochlear or anterior knee surgery.


Distal Femoral Physis



  • The distal femoral physis has an undulating pattern. The central ridge and the medial and lateral “valleys” change shape during growth (Figure 27.3). As the epiphysis starts cupping the metaphysis, the “nadir” on the medial and the lateral side deepens and the central ridge decreases in relative height. The metaphyseal-epiphyseal junctional notches are higher than the central ridge at skeletal maturity.19


  • Topographic and radiographic evaluation of the distal femoral physis shows it to have a cruciform pattern when viewed from above.19 The central ridge (from anterior to posterior), lateral ridge, and medial peak (from medial to lateral) divide the physis into four valleys: anteromedial, anterolateral, posteromedial, and posterolateral (Figure 27.4). The most inferior aspect of the physis on the medial side is “anteromedial” and the most inferior aspect of the physis on the lateral side is “posterolateral”20 (Figure 27.5). These positions are important to take into consideration when surgery is performed around the distal femoral physis.


MPFL Attachments and Distal Femoral Physis



  • Based on pediatric cadaveric study, the MPFL attached to the proximal two-thirds of the patella. On average, the center of MPFL insertion footprint was 4.7 mm above the equator of the patella and it spanned a mean 41% of the length of the patella.21


  • On the femur, the MPFL attached posterosuperior to the medial epicondyle and just distal to the adductor tubercle. The medial epicondyle, adductor tubercle, and MPFL femoral insertion are distal to the physis in most children and adolescents (Figure 27.6). With growth, the MPFL insertion moves closer to the distal femoral physis and may cross proximal to the physis (Table 27.2).






    Figure 27.4 Proximal view of three-dimensional reconstruction of CT scan of distal femoral physis of left knee. The central ridge, lateral ridge, and medial peak divide the physis into four quadrants/valleys. On the medial side, the anteromedial (AM) quadrant is deeper than the posteromedial (PM) quadrant. On the lateral side, the posterolateral (PL) quadrant is deeper than the anterolateral (AL) quadrant. From Liu RW, Armstrong DG, Levine AD, Gilmore A, Thompson GH, Cooperman DR. An anatomic study of the distal femoral epiphysis. J Pediatr Orthop. 2013;33(7):743-749, with permission.







    Figure 27.5 (A) Lateral and (B) anteroposterior view of knee shows the undulating nature of the distal femoral physis. The “nadir” of medial aspect of physis is anteromedial (yellow circle) and the lowest point of lateral aspect of physis is posterolateral (blue circle). The highest point of central ridge (green circle) is in the center of the physis on both views.






    Figure 27.6 A three-dimensional reconstruction of CT scan of right knee after anatomic medial patellofemoral ligament reconstruction shows the relationships between different landmarks and distal femoral physis. From A to F, the knee is rotated in 30 degrees increment. A, The yellow line represents the trajectory of femoral tunnel. The blue arrow shows the exit point of femoral guide pin. B, The white circle represents medial avulsion fracture of patella. The black dotted line shows that drilling parallel to the distal femoral physis would cause breach of physis. C-F, The femoral attachment point (black arrow), the adductor tubercle (red circle), and medial epicondyle (black circle) are below the level of the distal femoral physis. The patellar tunnel is seen (red arrow).









    TABLE 27.2 Relationship Between MPFL Femoral Insertion and Distal Femoral Physis





























































    Authors (Year)

    Study Type

    No.

    Average Age

    Average Distance Distal to Physis (Minus Indicates Proximal to Physis)

    Comments


    Shea et al (2010)39

    Lateral x-ray

    20

    12.7 y

    −2.7 mm (female)


    −4.6 mm (male)


    Kepler et al (2011)40

    MRI

    43

    14.3 y

    5 mm


    Nelitz et al (2011)41

    AP, lateral x-ray

    27

    14.3 y

    6.4 mm


    Greenrod et al (2013)25

    MRI

    159

    11 y

    10 mm

    Distance increased 0.6 mm/y


    Shea et al (2014)42

    Cadaver

    6

    1 m, 11 m, 11 m, 8 y, 10 y, 11 y

    <1 y: 9 mm Others: 4 mm


    Farrow et al (2014)43

    Cadaver

    16

    12 y

    8.5 mm

    Adductor tubercle and medial epicondyle distal to physis


    Shea et al (2018)44

    Cadaver

    36

    2-11 y

    3 mm (average for all specimens)


    <7 y: 4.7 mm


    >7 y: −0.8 mm

    MPFL origin distal and posterior for those <7 y of age; proximal and anterior for those >7 y


    Abbreviations: AP, anteroposterior; MPFL, medial patellofemoral ligament.



  • The MPFL femoral attachment point on a lateral radiograph has been well-described for adults by Schöttle et al.22 The same technique has been found to be reproducible in the skeletally immature.23 However, the femoral attachment point is in close relationship to the distal femoral physis and there is valid concern about physeal violation and growth disturbances during pediatric MPFL reconstruction. To be at a safe distance from the physis, the attachment point could be moved slightly distal; this could lead to changes in graft isometry. To maintain graft isometry, there have been recommendations to move the attachment point more anterior (Eric J. Wall, MD, personal communication) or even proximal to the physis24 (Figure 27.7).






    Figure 27.7 A, The Schöttle point on pediatric knee is marked (black circle). B, If the attachment site is moved distal (to stay clear of the distal femoral physis), it can lead to an anisometric position of the graft. The dashed arrow shows distance to proximal trochlea, the solid arrow shows distance to distal trochlea. C, If the femoral tunnel is moved distally, it should also be moved anteriorly to maintain an isometric position. D, Also, if the femoral tunnel is moved proximal to the physis, an isometric graft position could be obtained. The proximal position is not recommended due to potential migration of the graft away from the physis with growth.


  • Because the inferior-most part “nadir” of the medial femoral physis is usually lower than the MPFL femoral attachment point, a femoral tunnel parallel to the physis would likely cause physeal injury (Figure 27.8).


  • Based on MRI study, a 20-mm screw placed parallel to the physis violated the physis in 64% of patients. However, if the screw was directed 45° oblique distally, it was safe in 98% of patients25 (Figure 27.8).


  • If the femoral tunnel was angled less than 10° in the distal direction, it would violate the distal femoral
    physis. If it was angled less than 10° anteriorly, it would violate the intercondylar notch. If it was angled greater than 20° distally and anteriorly, it would violate the joint cartilage surface.26






    Figure 27.8 A, Anteroposterior view of the left knee shows three different trajectories of femoral tunnel, relative to distal femoral physis (dashed white line). B, Guide pin placed parallel to the distal femoral physis, as in 1, would breach the physis. C, Guide pin angled about 10° to 15° distal to the physis, as in 2, would be ideal. Line 3 represents a 45° oblique femoral socket, which is safe, but a pull-through technique cannot be used.


  • Thus, the optimal trajectory of the femoral tunnel should be angled 15° to 20° distally and anteriorly related to the distal femoral physis26 (Figures 27.6 and 27.8).


  • Injury to the distal femoral physis can lead to deformity or limb length discrepancy.27 In this regard, anterior cruciate ligament (ACL) surgery is safer than MPFL surgery because the trajectory of the femoral tunnel during ACL reconstruction is relatively perpendicular to the physis, whereas the trajectory of femoral tunnel during MPFL reconstruction is relatively parallel to the physis. Thus, technical errors during femoral tunnel placement for MPFL surgery could violate significantly more area of the distal femoral physis.


Classification



  • Several classification systems have been described for patellar dislocations based on clinical and radiographic findings. Most of these have been reported based on patellar instability in adults.


  • Parikh and Lykissas described different types of patellar instability in children and adolescents.28 The main purpose to differentiate varied patterns of instability was to help guide treatment as to which patients would benefit from an isolated MPFL reconstruction. The types of patellar instability and recommended treatment options are discussed in Chapter 3.


  • The indications and contraindications for physeal-sparing MPFL reconstruction are listed in Table 27.3.


EVALUATION


Patient History

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Dec 1, 2019 | Posted by in ORTHOPEDIC | Comments Off on Physeal-Sparing Medial Patellofemoral Ligament Reconstruction

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