Patellofemoral Anatomy and Its Surgical Implications

Medial Patellofemoral Anatomy

Proximal Medial Patellar Complex

As our understanding of medial patellofemoral anatomy continues to grow, the implications for surgical reconstruction have evolved. Whereas earlier reports focused on reconstruction of the medial patellofemoral ligament (MPFL) as the primary treatment for lateral patellar instability, more recent anatomical descriptions have noted additional fibres that extend proximal to the patella, leading to the development of new reconstruction techniques. Additional reports have focused on the significance of the fibres that are distal to the MPFL and contribute to the lateral restraint of the patella in knee flexion.

These anatomical studies have led to several new terms that are used to describe and classify the medial patellar stabilisers that include the MPFL. The proximal medial restraints are the MPFL and medial quadriceps tendon femoral ligament (MQTFL), which some authors also refer to in a combined fashion as the proximal medial patellofemoral complex (MPFC) because of the variability of the fibres at their attachment on the extensor mechanism.

The MPFL has traditionally been described as an extension of fibres from the medial femur to the medial border of the patella, in a broad, fan-shaped attachment spanning 14 to 41 mm. ^, ^, The ligament is thin, reported to be 0.44 ± 0.19 mm in one study, running in layer 2 of the knee, deep to the vastus medialis obliquus, while remaining superficial (extraarticular) in relation to the capsule. Additional proximal fibres of the MPFL extending to the quadriceps tendon have been highlighted by authors who have performed dissections using an intraarticular approach. Fulkerson termed these fibres as the MQTFL, describing the attachment point as distinct from the MPFL fibres that insert on the patella ( Fig. 26.1 ). Tanaka described the broad attachment to both structures as part of a single complex, showing in a cadaveric study of 28 knees that 57% ± 20% of the fibres attached to the patella, whereas the remainder of the fibres attached to the quadriceps tendon. The author also noted significant variability between the location of the attachments that may account for some of these differing descriptions, with some knees having all fibres attaching to the patella and others having all fibres attaching solely to the quadriceps tendon.

The biomechanics of the proximal structures to date have focused on the MPFL; the MPFL is agreeably the primary stabiliser resisting lateral patellar translation, contributing approximately 50% to 60% in early knee flexion. ^, ^, The literature also supports that the MPFL is injured in nearly all acute traumatic dislocations, with the patella insertion primarily involved in isolation or combination in the paediatric population. The biomechanics and injury pattern of the MQTFL portion of the proximal medial structures continue to be defined.

Surgical Correlation for Proximal MPFC Patellar Attachment Sites

The broad attachment of the fibres on the extensor mechanism allows for several options in terms of reconstruction. The proximal extent of the MPFC has been described to be 14.6 mm proximal to the superior pole of the patella, with the distal extent inserting 26.7 mm distal to the superior pole, indicating that anatomical reconstruction should maintain the fixation points on the extensor mechanism within these areas.

Kang et al. described two functional bundles within this complex, including the superior oblique bundle attaching to the proximal patella and the quadriceps tendon and the inferior straight bundle extending to the patella. The differential lengths of the proximal and distal fibres are reported to be 2 to 7 mm, ^, which suggest varying isometric functions between the two. This supports the concept of a 2019 report on a double bundle reconstruction technique that includes concurrent reconstruction of both MPFL and MQTFL fibres. In a two-arm MPFL surgical construct in a patient with a small patella or short articular length, the distal arm of the MPFL graft should not be positioned lower than 50% of the articular surface length. Therefore positioning one arm to the proximal patella and the superior arm to the quadriceps tendon would potentially be a better surgical construct ( Fig. 26.2 ).

Alternatively, understanding the midpoint of the anterior attachment can serve as a guide in the setting of reconstruction with single-stranded grafts. The midpoint of the MPFC at its anterior attachment has been reported to be reproducibly identified at the junction of the medial border of the quadriceps tendon and the articular surface of the patella in a cadaveric study. This point has also been shown to correlate radiographically with a location that is 19% of the patellar articular distance from the superior articular pole, which may be helpful when using fluoroscopic guidance to identify the midpoint of its patellar attachment.

Surgical Correlation of Proximal MPFC Femoral Insertion

On the femoral side, the MPFL and MQTFL originate from a common origin that has been described to be in the vicinity of the area between the medial epicondyle and the adductor tubercle, with additional soft tissue attachments extending to the adductor tendon and medial collateral ligament. ^, ^, The footprint of this ligament has been described to have an area of 26 mm and is ribbon shaped, ranging 9 to 17 mm in diameter. ^, The importance of understanding the precise origin of the MPFC stems from the fact that the relative position of the femoral tunnel can influence the function of the graft during reconstruction. Elias and Cosgarea have demonstrated in a computational model that 5 mm of malpositioning of the femoral tunnel can have deleterious impacts on graft forces. Nelitz et al. identified femoral tunnel malposition as an important factor leading to unsuccessful MPFL reconstruction results.

The use of radiographs can serve as a tool in the intraoperative assessment of femoral tunnel position. Schöttle has described the midpoint of the native origin of the MPFL to be at a point on lateral radiographs that is 1.3 mm anterior to the posterior cortical line, 2.5 mm distal to the proximal origin of the posterior medial femoral condyle and proximal to the Blumensaat line ( Fig. 26.3A ). Stephen et al. also described this radiographically relative to the size of the femur, noting that the MPFL origin is 60% of the anteroposterior dimension from the anterior surface and 50% from the distal condylar surface ( Fig. 26.3B ). Ziegler et al. have emphasised the importance of obtaining proper lateral views during radiographic assessment of MPFL tunnel position because they demonstrated that 5 degrees of posteroanterior rotation can lead to a difference of 9.2 mm in femoral tunnel position when solely relying on radiographic landmarks. When using fluoroscopy, the MPFL in about 70% of patients will have a favourable metric behaviour (nearly isometric) between extension and 30 to 40 degrees of flexion, or slightly looser during maximum extension (<3 to 4 mm) and slight looser beyond 30 to 40 degrees of flexion (<3 to 4 mm). ^, ^, Therefore after initially localising the femoral tunnel point with the fluoroscopy, one should test the ligament behaviour during range of motion (ROM) and make necessary changes to obtain a favourable metric behaviour. When the metric behaviour is not ideal after defining the point with the fluoroscopy, the most commonly needed change is to move distal or posterodistal.

Distal Medial Patellar Complex

The distal medial patellar restraints include the medial patellotibial ligament (MPTL) and medial patellomeniscal ligament (MPML), which originate on the medial patella, distal to the MPFL attachment. The MPTL and MPML have a common insertion on the distal medial patella over an area of 27.4 mm ² , approximately 3.5 mm medial and 3.5 mm proximal to the medial border of the proximal patellar tendon origin ( Fig. 26.4 ). Both structures course distally, superficial to the capsule, inserting on the tibia (MPTL) and the meniscus (MPML).

The MPTL is 35 to 50 mm long and 4 to 22 mm wide. ^, ^, It maintains a more vertical orientation than the MPML and inserts on the medial tibial tubercle, at a location 1.5 cm distal to the joint line on the anteromedial tibia. ^, ^, Radiographically the insertion of the MPTL has been correlated to a point 5 to 10 mm distal to the tibiofemoral joint line on the anteroposterior view and 9 to 13 mm distal to the tibial slope line on the lateral view. ^,

The MPML is more horizontally oriented than the MPTL. From its origin on the distal medial patella, it courses posteriorly to insert with a broad attachment on the medial meniscus. It is a round and cordlike ligament within the deep capsular layer, having been described as 20 to 40 mm long and 3 to 10 mm wide. ^, ^, ^, Hinckel et al. have noted that the location of the meniscal attachment was variable. In their series of nine cadaveric dissections, the MPML was found to attach to the anterior horn in seven knees and the junction between the anterior horn and meniscal body in the remaining two knees. The angulation of the MPTL in relation to the patellar tendon is 18 to 22 degrees with the knee in 90 degrees of flexion; the MPML angulation in relationship to the patellar tendon is 22 to 42 degrees with the knee in 90 degrees of flexion. ^, ^,

From a biomechanical standpoint, the distal medial patellar restraints have been described as having a role in restricting lateral patellar translation and lateral patellar tilt in deeper degrees of knee flexion, in contrast to the proximal restraints which function primarily in early knee flexion. ^, ^, ^, These authors’ biomechanical study revealed that MPTL and MPML have an increased role in restriction of lateral translation, lateral patellar tilt and patellar rotation at 90 degrees of flexion compared with full extension, demonstrating an important stabilising role particularly at higher knee flexion angles. In addition, clinically, isolated lesions of the MPML have been associated with subluxation in terminal extension without frank dislocation. Furthermore, the MPTL has been found to have similar or higher mean stiffness and failure loads compared with the MPFL, ^, which indicated the MPTL may also have an important functional role for surgical medial patellar stabilisation.

The biomechanical role of the distal MPFC continues to evolve. Most biomechanical formats studying the role of medial ligaments in lateral patella dislocation have used straight lateral translation ^, ^, without consideration of the probable superior and superolateral forces on the patella in vivo. Indeed, the anatomical position of the MPTL/MPML suggests a role in resisting a more patellar superolateral directed force.

Surgical Correlation

The MPTL reconstruction and analogous procedures have been part of the surgical armamentarium for surgical treatment of patella instability for decades; indeed, in 1922, Galeazzi described a semitendinosus patellar tenodesis for treating patellar instability. A 2018 systematic review of MPTL reconstructions found a considerable number of articles ( N = 19) and number of knees ( N = 403), with many studies preceding the widespread clinical use of MPFL reconstruction. The systematic review reported favourable outcomes with low rates of patellar redislocation, though the quality of the articles was variable.

Though historically MPTL surgical reconstructions varied in the location of the MPTL tibial insertion, the surgical descriptions typically were focused more on restoring the angulation of the MPML (22 to 45 degrees), which would be better positioned to resist lateral patella translation when they were performed in isolation (not in addition to the MPFL). Therefore those MPTL reconstructions may be more in alignment with the MPML, with a tibial-based insertion. However, in more recent studies the MPTL reconstruction is performed in addition to the MPFL and there are reports of anatomical MPTL reconstruction. Thus one can make a choice on whether to use the reconstruction of the distal medial patellar complex to resist superolateral forces (closer to MPTL direction, more lateral and distal tibial insertion) or majority lateral forces (closer to MPML direction, more medial and proximal tibial insertion). Further investigations will help develop the ideal surgical construction for best functional outcomes.

The introduction of MPFL reconstruction resulted in a shift from the reconstruction of the MPTL to the MPFL, in part because of its favourable biomechanics in resisting lateral translation at near full knee extension. ^, ^, Interest in the MPTL as an augment to MPFL has grown, in part fuelled by a 12% persistence of objective or subjective instability after isolated MPFL reconstruction. By providing additional ligamentous support to resist lateral patellar translation, the combination of the reconstruction of a distal medial patella restraint to the MPFL can potentially improve the outcomes relative to isolated MPFL reconstructions. An additional restraint may decrease the need for additional bony procedures by defining a different surgical threshold for adding a bony procedure to a patellar stabilising procedure against lateral patellar dislocation, thereby reducing surgical morbidity.

Further studies are needed to elucidate the clinical and biomechanical roles of these additional medial patellar stabilisers and to guide advances in anatomical soft tissue reconstruction in the treatment of patellofemoral instability. However, a prudent approach, based on studies to date, suggests the addition of an MPTL in the following clinical conditions:

1.

flexion instability (obligate dislocation in flexion)
2.

extension patellar subluxation (resisting superolateral patellar translation)
3.

knee hyperextension (helping to resist ‘functional patella alta’)

Lateral Patellofemoral Complex

The lateral soft tissue patellofemoral joint complex is composed of the iliotibial band extension to the patella (ITB–patella), the vastus lateralis, and the lateral patellofemoral ligament (LPFL), lateral patellotibial ligament (LPTL) and lateral patellomeniscal ligament (LPML), with intimate connections among these structures. The complex has superficial longitudinal fibres (superficial fibres of the ITB–patella) and deep transverse fibres (deep fibres of the ITB–patella, vastus lateralis, LPFL, LPTL and LPML). The patellar attachment of the lateral patellofemoral complex (LPC) was observed to attach mainly in the middle one-third of the patella; however, it often extended distally into the proximal patellar tendon ( Fig. 26.5 ). The LPFL has an average length of 23 mm and average width of 13 to 16 mm and typically spans 45% of the lateral patellar width. The femoral insertion of the LPFL to the epicondyle is variable, though the most common insertion pattern is a broad figure 8 anterior to the lateral femoral epicondyle.