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Variability in tibial tubercle to trochlear groove distance and measurements of malalignment 

Patellofemoral Update

In the assessment of patellar instability, malalignment is an important factor to consider in determining the appropriate surgical treatment. Mechanical variations, such as excessive femoral anterversion, genu valgum or external tibial torsion, can increase the lateralization of the tibial tubercle and contribute to patellar instability. Identifying such factors can aid in determining the optimal surgical options for a patient, particularly with respect to tibial tubercle osteotomy. Realigning the extensor mechanism through this procedure can decrease lateralizing forces on the patella and minimize stress on its soft tissue restraints.

Tibial tubercle osteotomy is not without risks, however. Complications such as fractures, nonunion and deep venous thromboses have been reported in this patient population, and the rehabilitation course is longer and more restrictive than in soft tissue procedures alone. Understanding the reliability and variability in the current methods of assessing malalignment can help guide a surgeon’s indications for tubercle osteotomy in the treatment of patellar instability.

Quantify malalignment

Q angle measurement is one way to quantify malalignment through clinical assessment. Originally described by Brattstrom, it measures the angle between one line through the center of the patella to the anterior superior iliac spine, and a second line through the center of the patella to the tibial tubercle. While a Q angle greater than 20° is considered to be pathologic, normal reported values have varied widely, and there exists considerable variation in interobserver techniques and reliability. Furthermore, lateral subluxation of the patella during evaluation may skew this measurement, leading to misleadingly low angles. Measurement of Q angle in 90° of knee flexion, when the patella is fully engaged within the trochlear groove, can minimize this occurrence.

Tibial tubercle to trochlear groove (TTTG) distance is a radiographic measurement that is often used to quantify malalignment on axial imaging.Originally described on radiographs and CT, and later on MRI, this measurement quantifies the lateral distance from the trochlear groove to the tibial tubercle, denoting the pull of the extensor mechanism lateral to the trochlear groove. In patients with symptomatic patellar instability, TTTG values greater than 15 to 20 mm have been reported as a potential indication for tibial tubercle osteotomy.

However, large variations in normal values have been reported in TTTG distance, making it difficult to define clear indications for tubercle osteotomy.Some have reported poor interrater reliability, whereas others have pointed out differences in measurement conditions. Schoettle and colleagues compared TTTG measurements on both CT and MRI and noted two distinct measurement techniques, using bony landmarks vs. cartilaginous-tendon, or “functional” landmarks.  On the trochlear side, the bony TTTG measurement references the deepest portion of the bony trochlear groove, as described by Goutallier, while the cartilaginous-tendon TTTG measurement uses the deepest portion of the cartilaginous trochlear groove.

On the tuberosity side, the bony landmark references the anterior-most point of the tibial tuberosity as described by Goutallier, while the cartilaginous-tendon TTTG references the central portion of the patellar tendon attachment. While Schoettle did not find a significant difference between the two methods, our recent study using kinematic CT imaging found that the TTTG distance referencing the center of the tibial tubercle was 4.2 mm greater than the TTTG distance measured utilizing the bony landmark described by Goutallier. This difference in measurement led to considerable variability in the indicated surgical treatment our study population, with only 42% of 38 knees having an abnormal TTTG (greater than 15 mm) using the bony prominence, vs. 79% in the same population using the central portion of the patella tendon.

Flexion angle

Recent studies have shown that TTTG distance can vary considerably based on the flexion angle at which the knee is imaged. Dietrich and colleagues found more than 5-mm difference in TTTG between 0° and 15° of knee flexion in an MRI study of asymptomatic knees. A recent study by our group found similar results in a CT study of symptomatic knees. Dietrich and colleagues speculated these changes in TTTG distance with knee flexion may be related to the rotational changes that occur with the screw home mechanism. As knee positioning and flexion angle can often vary during CT or MR imaging, knee flexion angle should be assessed on sagittal images and be taken into consideration when using TTTG distance as a possible indication for tibial tubercle osteotomy.

In addition to knee flexion angle, Izadpanah and colleagues recently reported the effect of weight bearing and quadriceps activation on variability of TTTG distance, noting a decrease by more than 5 mm with weightbearing in extension vs. supine imaging. As our technological capacity for imaging evolves, the applicability of conventional radiologic measurements may continue to adapt as well. A recent study by Seitlinger proposed the tibial tubercle to posterior cruciate ligament (TT-PCL) distance as an alternative measurement to TTTG distance. This limits both anatomic reference points to landmarks on the tibia in order to avoid the variability noted with knee flexion. The authors report a value of 24 mm or greater to be abnormal. Further studies are needed to determine the benefit of utilizing TT-PCL distance in the treatment of patellar instability.

Tubercle lateralization

The difficulty of assessing the value of TTTG distance, or any measurement of tubercle lateralization, as a precise indication for tibial tubercle osteotomy lies in the multifactorial nature of patellar instability. As patients can additionally have varying amounts of trochlear dysplasia, patella alta and/or medial patellofemoral ligament insufficiency that may contribute to their symptoms, our ability to isolate and study the role of TTTG distance in the diagnosis and treatment of patellar instability has been limited. Several biomechanical studies have shown the relationship between TTTG distance and lateral patellar position and tilt, and correction of these parameters with tibial tubercle osteotomy has been noted in these studies. An exact algorithm to guide surgical treatment, however, remains yet to be determined.

When considering tibial tubercle osteotomy in a patient with patellar instability, a TTTG distance greater than 15 mm to 20 mm can be used as a guideline, with consideration of the factors that can affect this number.Measurement technique and knee flexion angle at the time of imaging should be considered when assessing TTTG distance. Furthermore, tuberosity lateralization should be considered in the context of the multiple other factors that may contribute to instability in the patient, and those elements should be addressed accordingly. Further studies and measurement techniques to quantify malalignment are needed, as well as standardization of the indications for tibial tubercle osteotomy in the treatment of patellar instability.

References:

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Tanaka MJ. Correlation between changes in TTTG distance and patellar position during active knee extension on dynamic kinematic CT imaging. To be presented at: Arthroscopy Association of North America Annual Meeting, May 1-4, 2014, Hollywood, Fla.

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• Miho J. Tanaka, MD, is associate team physician, St. Louis Cardinals, and director, Women's Sports Medicine Initiative, Regeneration Orthopedics in St. Louis.
• Disclosure: Tanaka has no relevant financial disclosures.