Page 12 - JCTR-11-3

Journal of Clinical and
Translational Research Lateral patellar instability in deep flexion

Table 2. The biomechanical sequence of lateral patellar tomography scan (CT) and 8.9 – 11.1 mm on magnetic
51
instability in deep flexion resonance imaging (MRI). 40,53 Cutoff values commonly
measured in extension more than 20 mm are considered
Angle Biomechanical sequence
pathologic. 40,54-56 A TT-PCL distance of <24 mm is
<45° (stable • The lateral facet of the trochlea is the most prominent, considered normal. 55
patella) and the inclination is high.
• The quadriceps tendon and patellar tendon pull almost However, the angle of knee flexion during imaging plays
in the opposite direction. The patellofemoral (PF) joint a critical role in accurately measuring both the TT-TG
reaction force (JRF) is low.
• The iliotibial tract, with its connections to the lateral and TT-PCL distances for assessing patellar instability.
patellar structures, runs anterior to the rotational axis, Both TT-TG and TT-PCL distances increase significantly
resulting in a low laterally oriented force vector. during the final knee extension due to the screw-home
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>45° • The medial facet of the trochlea becomes more, and mechanism. 57,58 Tanaka et al. described that the TT-TG
(dislocating the lateral facet becomes less prominent with lowered distance decreased by approximately 1 mm with each 5°
patella) inclination, resulting in decreased lateral resisting increase of knee flexion between 5° and 30° in patients
forces. 40,57
• The lateral femoral contact point rolls back, causing an with patellar instability. Other authors confirmed that
external rotation of the femur. the TT-TG distance assessed in the axial plane decreased
• The iliotibial tract glides posteriorly across the lateral with greater flexion. 6,40,59 As knee flexion occurs, tibial
femoral condyle, changing the resultant force vector internal rotation and tibial tubercle medialization reduce
and exerting a posterolateral force on the patella. the lateral force vector of the quadriceps angle. The
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• The smaller upper part of the patella has increasing
contact with the terminal sulcus/false groove/ lateral position of the tibial tubercle, as a relevant factor
dysplastic lateral femoral condyle, and the for patellar instability in extension, decreases with deeper
osteochondral stability decreases. knee flexion. 40-42 Thus, the anatomic location of the tibial
• The tension in the quadriceps tendon becomes higher tubercle does not appear to be a significant factor in
than in the patellar tendon, resulting in a higher PF patellar instability during deep flexion.
JRF.
• Contractures/fibrosis of lateral soft‑tissue structures In summary, these biomechanical and clinical
and quadriceps tendon cause increased laterally reflections indicate that other etiological pathologies
oriented forces acting on the patella.
associated with lateral patellar instability in deep flexion
should be considered when determining appropriate
decreased patellotrochlear cartilage overlap. 9,16,52 As a surgical treatment.
result, the patella alta prevents proper engagement of the
proximal trochlea during extension and early flexion. 3.4. Clinical evaluation
Therefore, patella alta is considered a potential risk factor This rare lateral patellar instability in deep flexion can be
for patellar instability close to extension. identified through physical examination and confirmed
2
Biomechanically, the flexion angle at which a patella alta with radiographs, CT, and MRI. This condition typically
16
becomes engaged by the trochlear groove is increased. In begins at a younger age and is often well tolerated for an
deep flexion, the patella is more securely engaged in the extended period. 4,5,60 Over time, however, dysfunction and
trochlear groove, enhancing stability. The patella only instability may result in difficulties with daily activities and
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5,60
contacts the terminal sulcus at very high flexion, not at the running. A comprehensive history, including various
angles where flexion instability occurs. In summary, patella symptoms, unsuccessful treatments (whether conservative
alta is not a biomechanically significant factor contributing or surgical), functional disability, and a thorough physical
to patellar instability in deep flexion. examination, are essential factors for diagnosis.
3.3.3. Lateralization of the tibial tubercle 3.4.1. Clinical assessment
Excessive lateralization of the tibial tubercle is considered The patellofemoral joint and surrounding soft tissue
a major predisposing factor for patellar instability. Two structures are thoroughly examined, including instability
common measurements used to assess the position of the tests, tightness, patella gliding during the whole range of
tibial tubercle are the TT-TG distance and the distance motion, muscle conditions, and contractures. Typically,
from the center of the patellar tendon attachment on the the patella escapes laterally only in deep flexion beyond
tibial tuberosity to the medial border of the posterior 45° (Figure 5A). Close to extension, the patella is stable
2,5
cruciate ligament attachment on the tibia (TT-PCL). 40,51,53,54 and well-engaged (Figure 5B). The patella subluxates
The normal range of TT-TG values measured in extension or dislocates laterally each time the knee is flexed, but
has been reported to be 9.3 – 16.1 mm on computed full flexion can still be achieved when the patella is

Volume 11 Issue 3 (2025) 6 doi: 10.36922/jctr.7131