Tibial Tubercle Distalization

Scott Smith

David C. Flanigan

Robert A. Magnussen

INTRODUCTION

Pathogenesis

Recurrent patellar instability predominately affects young, active people and has been attributed to a combination of patient anatomic factors, including trochlear dysplasia, medial patellofemoral ligament (MPFL) injury/redundancy, increased tibial tubercle-trochlear groove (TT-TG) distance, and patella alta.
Patella alta has been an underappreciated contributor to patellar instability, even though it has been reported to be present in up to 24% of recurrent dislocators¹ and is associated with recurrent instability after previous treatment with isolated MPFL reconstruction.²,³
It is thought that the mechanism for increased instability in the setting of patella alta is delayed engagement of the patella with the bony constraints of the trochlear groove, allowing increased lateral translation of the patella in deeper flexion.⁴ Although the underlying etiology is unclear, an increased patellar tendon length, rather than a proximal insertion onto the tibia, has been identified as an underlying cause of patella alta.⁵

The effects of patella alta on stability are compounded by trochlear dysplasia, increased TT-TG distance, and MPFL insufficiency.

TABLE 16.1 Description of Patellar Height Indices

Index	Measurements	Normal Ratios
Insall-Salvati ratio	Length of patellar tendon/longest sagittal diameter of the patella	0.8-1.2
Modified Insall-Salvati Index	Length of patellar tendon/longest sagittal diameter of patellar articular surface	<2
Blackburne-Peel index	Length of patellar articular surface/height of lower pole articular surface above tibial plateau line	0.5-1
Caton-Deschamps index	Distance between patellar articular surface and anterosuperior border of the tibia/length of articular surface of the patella	0.8-1.2

One aspect of the treatment of recurrent patellar instability in the setting of patella alta, presented in this chapter, is osteotomy and distalization of the tibial tubercle. Osteotomy and medial or anteromedial transfer of the tibial tubercle is often used in the setting of an increased TT-TG distance, with the goal of improving patellar tracking and offloading the lateral facet. By performing a distalization procedure, the goal is to attain a more normal patellar height and improve tracking by allowing earlier engagement of the patella within the trochlear groove as well as possibly offloading the distal patellar articular cartilage.⁶

Classification

There have been multiple measurements of patella alta described in the literature, many of which can be performed on a lateral radiograph of the knee (Table 16.1).
There is significant variation between measurements and categorization of normal, alta, and baja despite good interobserver reliability of each ratio.⁷

The authors prefer to use the Caton-Deschamps (CD) index because unlike the Insall-Salvati ratio, it can be utilized postoperatively to measure the degree of distalization performed and resultant patellar height.

TABLE 16.2 Indications and Contraindications of Tibial Tubercle Distalization

Indications	Contraindications
Recurrent lateral patellar dislocations with failed nonoperative treatment and patella alta (Caton-Deschamps [CD] index > 1.2, patellotrochlear index [PTI] < 0.15)	Patella baja Patients with inability to protect weight bearing

Measurements utilizing magnetic resonance imaging (MRI) have also been described. MRI is a commonly utilized tool to evaluate the health of the chondral surfaces and stabilizing structures and has the advantage over x-ray when evaluating the geometry of the articular cartilage and subchondral bone.
The patellotrochlear index (PTI) is defined as the ratio of the length of the trochlear articular surface that overlaps the patellar articular cartilage to the length of the patellar cartilage on a single sagittal MRI slice.⁸ PTI is a beneficial adjunct to traditional height measures because it can help identify patients with functional patella alta in the setting of a borderline CD index. Patients with less patellar/trochlear overlap are more likely to have delayed articulation between the two surfaces during initiation of flexion.⁹
Table 16.2 lists indications and contraindications for tibial tubercle distalization.

EVALUATION

Patient History

A comprehensive history is required when evaluating a patient with patellar instability. It is important to elicit the chronicity of the patient’s complaints.
Patients with an acute injury with an otherwise normal past history often present after a traumatic event, usually with significant pain and possibly hemarthrosis.
It is important to determine the number of prior dislocations as well as the inciting events because those with recurrent instability often have instability events with increasing frequency and minimal trauma.
Those with recurrent instability often report subjective instability or persistent pain. Mechanical symptoms from loose bodies may also be present.

Physical Examination and Findings

Physical examination of the patient begins with an assessment of overall limb alignment, particularly for the presence of significant genu valgum and an increased Q-angle.
The knee should be evaluated for the presence of an effusion and palpated for sources of pain, particularly along the medial stabilizing structures.
A thorough ligamentous examination should be performed.
Attention should then be turned to the patella. Patellar tracking should be assessed with active knee extension. Crepitus suggests patellofemoral chondral damage. A J-sign, or lateral movement of the patella as it exits the trochlear groove, is indicative of some combination of trochlear dysplasia, tibial tubercle lateralization, patella alta, and tight lateral structures.
The amount of lateral translation of the patella, as well as patient apprehension, should be determined. This maneuver should be performed at varying degrees of flexion. The apprehension should improve with increased flexion as the patella engages the trochlea, usually between 30° and 40°. Patients with patella alta, owing to delayed entrance of the patella within the groove, may demonstrate apprehension at greater degrees of flexion.

Imaging

Initial evaluation of the patient with patellar instability should include radiographs of the involved knee. The authors routinely obtain full leg length, weight-bearing posteroanterior, lateral, and axial views.
In evaluating patella alta, attention should be turned to the lateral view, and the patellar height can be measured using the methods discussed previously (Figure 16.1).
An MRI should also be performed, both for the purposes of evaluating the chondral surfaces for damage and the geometry of the patellofemoral cartilage. The PTI can be measured using sagittal sequences (Figure 16.2).

Figure 16.1 Lateral radiograph of patient with patella alta. The Caton-Deschamps index is 1.56, consistent with patella alta.

Figure 16.2 Sagittal magnetic resonance imaging with measurement of patellotrochlear index. The index is 0.12, consistent with patella alta.

SURGICAL MANAGEMENT

Preoperative Planning

Care must be taken to evaluate all anatomic factors contributing to patellofemoral instability and address each at the time of surgery.
Regarding distalization, care must be taken to not overdistalize the tubercle as this would place more strain on the repair and increase the chances of nonunion and patella baja with subsequent pain and loss of flexion. There has also been concern about possibly increasing patellofemoral contact pressures after tubercle distalization, which could lead to arthritic changes,¹⁰ but these biomechanical studies were performed in cadaver with normal patellar height, not patella alta.
The authors prefer to distalize the tubercle to obtain a postoperative CD index of about 1.1 in order to prevent overcorrection and the development of patella baja.

Surgical Technique

Positioning