Elevated Patellofemoral and Tibiofemoral T1ρ Relaxation Times Following a First Time Patellar Dislocation

Abstract

Objective: The study was performed to evaluate cartilage within the knee following a first-time patellar dislocation, using elevated MRI-based T1ρ relaxation times as an indicator of low proteoglycan concentration. The hypothesis is that MRI-based T1ρ relaxation times for patellofemoral and tibiofemoral cartilage are significantly longer for knees being treated for patellar dislocation than for healthy control knees.

Design: Twenty-one subjects being treated for a first-time, unilateral dislocation of the patella and 16 healthy controls participated in MRI-based T1ρ relaxation time mapping. Mean relaxation times were quantified for patellofemoral and tibiofemoral regions for injured knees, the contralateral knees, and healthy controls. T1ρ values for each region were compared between the 3 groups with generalized estimating equations. Linear regressions were also performed to correlate T1ρ relaxation times with time from injury.

Results: The knees with a disloction had longer T1ρ relaxation times than the contralateral knees and control group at the medial patella and longer relaxation times than the control group at the lateral tibia (P < 0.05). T1ρ relaxation times at the medial patella also decreased with time from injury (r² = 0.21, P = 0.037).

Conclusions: Compositional changes to cartilage on the medial patella are related to traumatic impact during a dislocation. Potential exists for cartilage properties at the medial patella to improve with time. Cartilage degradation at the lateral tibia is not directly related to traumatic impact. The current baseline data are a starting point to characterize the pathway from a first-time dislocation to progressive cartilage degradation and osteoarthritis.

Keywords: T1ρ; cartilage; patellar dislocation.

Figure 1.

Measurements used to characterize knee anatomy. The lines drawn to calculate the Caton-Deschamps index (A) represent the distance from cartilage on the patella to the superior-anterior tibia and the superior-inferior length of the patellar cartilage. Lateral trochlear inclination (B) is the angle between a line along the lateral ridge of the trochlear groove and a line parallel to the posterior condylar axis of the femur. The posterior condylar axis is identified on a separate slice with the largest anterior-posterior distance of the femoral condyles. TT-TG distance (C) is the distance from the most prominent point on the tibial tuberosity to the deepest point of the trochlear groove along the posterior condylar axis. TT-TG = tibial tuberosity to trochlear groove.

Figure 2.

A sagittal view showing separation of cartilage on the femur into trochlear groove and femoral condyle regions based on a projection of Blumensaat’s line (A). Axial views show separation of cartilage on the patella into a central region centered on the patellar ridge, plus medial and lateral regions (B), and separation of cartilage in the trochlear groove into a central ridge centered on the deepest point of the trochlear groove, plus lateral and medial regions (C).

Figure 3.

T1ρ relaxation times mapped to patellofemoral and tibiofemoral cartilage from a Dual Echo Steady State scan for injured and uninjured knees. The images give examples of elevated relaxation times for cartilage at the medial patella and lateral tibia for an injured knee.

Figure 4.

T1ρ relaxation time at the medial patella versus the number of days since dislocation. The decrease in T1ρ relaxation time with days since injury was statistically significant (P = 0.037).