Operative Options for Extensor Mechanism Malalignment and Patellar Dislocation

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Chapter 39 Operative Options for Extensor Mechanism Malalignment and Patellar Dislocation

Frank R. Noyes, MD, Sue D. Barber-Westin, BS

INDICATIONS

Disorders of the patellofemoral joint are one of the most common causes of anterior knee pain related to inflammation of the parapatellar soft tissues, articular cartilage damage, and instability (subluxation, dislocation). Although the majority of patients respond favorably to conservative measures, surgical treatment is required for recalcitrant cases with distinct anatomic abnormalities that require correction. The key to the indications for surgical treatment is the diagnosis of the specific anatomic defects that cause the patient’s symptoms. This underscores the importance of the history and physical examination discussed later in this chapter.

The terminology used to describe patellofemoral disorders in the literature can be confusing. Patellar malalignment may be defined as a translational or rotational deviation of the patella relative to any axis. It is caused by an abnormal relationship between the patella, the soft tissues surrounding the patella, and the femoral and tibial osseous structures. The source of such abnormal patellar kinematics may be peripatellar tissue tightness or laxity; osteochondral dysplasias, such as a shallow or convex trochlear groove; bony abnormalities of the patella; rotational malalignment of the femur and tibia proximal and distal to the knee joint; an excessive proximal or distal position of the patella relative to the trochlea (patella alta and patella baja); and inflexibility or weakness of the quadriceps, hamstrings, and iliotibial band (ITB).

Many surgical procedures have been described for realignment of the patellofemoral mechanism including proximal realignment procedures, distal realignment procedures, or a combination of both. Proximal realignment procedures alter the medial-lateral position of the patella through balancing of soft tissue restraints proximal to its inferior pole. Included in this category are lateral retinacular release, medial retinacular capsular and medial patellomeniscal ligament (MPML) plication, vastus medialis obliquus (VMO) advancement, and medial patellofemoral ligament (MPFL) repair or reconstruction. Distal realignment procedures modify the medial-lateral, anterior-posterior, and proximal-distal positions of the patella by transfer of the tibial tubercle. Included in this category are anterior (Maquet ⁵⁵), medial (Elmslie-Trillat⁹³), and anteromedial (Fulkerson³³) transfer of the tibial tubercle. Literally hundreds of articles have been written on these operative procedures regarding their indications, technique, and clinical outcome and the reader is referred to recent review articles for further information.^2–4,^27,^31,^32,^36,⁶¹

The purpose of this chapter is to provide an algorithm of the surgical treatment of specific patellofemoral disorders. Because this is an evolving surgical field, the authors recognize many approaches and strategies to the treatment of these problems are published in the review articles cited previously. In this chapter, the technique of a proximal and distal patellar realignment, a modification of Elmslie-Trillat procedure, is described. It includes an arthroscopic lateral release if a contracted lateral retinaculum is present, a modified VMO advancement, and a modified medial transfer of the tibial tubercle. The goals of this procedure are to release abnormal lateral tethering tissues when present, provide a balanced medial tissue-ligament complex, and realign the quadriceps–patella–tibial tubercle relationship. The procedure is performed using cosmetically appealing incisions with rapid rehabilitation, immediate knee motion, early weight-bearing, and early return to function.

Critical Points INDICATIONS

• Majority of patients with patellofemoral disorders respond favorably to conservative measures.

• Surgical treatment required for recalcitrant cases with specific anatomic abnormalities that require correction.

• Patellar malalignment: a translational or rotational deviation of the patella relative to any axis.

• Proximal realignment procedures alter the medial-lateral position of the patella through balancing of soft tissue restraints proximal to its inferior pole.

• Distal realignment procedures modify the medial-lateral, anterior-posterior, and proximal-distal positions of the patella by transfer of the tibial tubercle.

• Operative procedures to be described

Modification of Elmslie-Trillat procedure: arthroscopic lateral release if a contracted lateral retinaculum is present, a modified VMO advancement, and a modified medial transfer of the tibial tubercle.

MPFL reconstruction using the quadriceps tendon based on the proximal patella, combined with a distal realignment procedure when indicated.

Correction of patella alta.

Iliotibial band lengthening.

• Types of extensor mechanism problems

Diffuse anterior knee pain without distinct anatomic abnormalities in alignment, subluxation, or joint damage: treated conservatively.

Anterior knee pain with mild to moderate abnormalities: treated conservatively.

Patellofemoral pain with definite anatomic signs of an extensor mechanism malalignment: candidates for surgery.

• First-time patella dislocation treatment categories

Anatomic abnormalities, congenital laxity MPFL, increased Q-angle, trochlear dysplasia: treated conservatively, candidates for surgery if symptomatic.

± underlying anatomic abnormalities, major joint hemarthrosis, MRI evidence of patellar or femoral osteochondral fracture: surgery, fixation fracture, MPFL repair.

No anatomic abnormalities, acute MPFL attachment tear: operative repair.

MPFL, medial patellofemoral ligament; MRI, magnetic resonance imaging; VMO, vastus medialis obliquus.

The second operative procedure described in detail is an MPFL reconstruction using the quadriceps tendon (QT) based on the proximal patella. This procedure is performed through a limited cosmetic incision and avoids harvesting the semitendinosus tendon or bony fixation at the patella or medial femur. This procedure has proved to be useful in revision cases that failed after a prior proximal realignment in which an inadequate MPFL was not addressed. The operative procedure is combined with a distal realignment when indicated, as is discussed. Other operative procedures the senior author has found useful for correcting patella alta and ITB lengthening are also described. Surgical procedures for rotational malalignment of excessive femoral anteversion and external tibial torsion are discussed in Chapter 40, Patellofemoral Disorders: Correction of Rotational Malalignment of the Lower Extremity, and not repeated here.

The majority of patients with patellofemoral malalignment respond favorably to conservative treatment programs, covered in detail in other publications.^17,^18,⁹⁶ In general, three types of extensor mechanism problems present for treatment. The first group of patients have diffuse anterior knee pain without distinct anatomic abnormalities in alignment, subluxation, or joint damage that can be identified. Commonly, these are younger athletes who are involved in multiple sports, and frequently, the only finding is the presence of tenderness of peripatellar soft tissues and the anterior fat pad tissues. On occasion, a mild joint effusion may be present. Authors have stressed that these symptoms frequently represent an overuse condition from excessive participation in athletics.^45,⁹¹ Treatment consists of allowing the soft tissues time to heal and inflammation to recede by modification of athletics, followed by appropriate strengthening and stretching of the muscle groups. Specific factors that may contribute to the symptomatic state include excessive conditioning, type of sport, and duration and frequency of athletic participation. Recurrence of symptoms is not an indication for surgical treatment, because distinct anatomic abnormalities of the extensor mechanism are not present. A cartilage-sensitive magnetic resonance imaging (MRI) may be helpful in recurrent symptomatic knees to determine the status of the patellofemoral joint and the presence of early articular cartilage damage that is aggravated by vigorous athletic activities.

There is a second group of patients with anterior knee pain who are similar in all aspects to the first group described previously, except that mild to moderate abnormalities are detected on the physical examination. For example, there may be a lateral subluxation (lateral glide) up to 50% of the patellar width and a mild to moderate increase in the Q-angle. There may be a physiologic posterolateral ligament laxity allowing increased external tibial rotation during activities (dynamic Q-angle). These patients are also treated conservatively and are not candidates for extensor mechanism realignment surgery.

The third group of patients comprises those who demonstrate definite anatomic signs of an extensor mechanism malalignment who are candidates for surgery. An example is those patients with symptomatic lateral patellofemoral subluxation events that do not respond to conservative treatment and have a deficient MPFL. Included in this group are patients who have had a prior patellofemoral dislocation and remain symptomatic, with either recurrence of lateral patellofemoral subluxation or a repeat dislocation event.

Nonoperative treatment of first-time patella dislocations has a re-dislocation rate that ranges from 14% to 57% in adult populations ^15,^30,^40,^50,^52,^53,⁹² and from 36% to 71% in pediatric populations.^12,⁷² Stefancin and Parker⁸⁷ conducted a systematic review of 70 articles that focused on the management of traumatic first-time patellar dislocations and concluded that conservative treatment was indicated initially except in specific circumstances. These included the finding of an osteochondral fracture, a substantial disruption of the MPFL, a laterally subluxated patella with normal alignment of the contralateral knee, or after failed conservative treatment or recurrent dislocations.

Patients with an acute lateral first-time dislocation are separated into three categories:

1 Dislocation with underlying anatomic abnormalities such as congenital laxity of the MPFL, increased Q-angle, and trochlear dysplasia, with similar abnormalities present in the opposite extremity. These patients undergo conservative treatment and are candidates for surgery if symptoms occur in the future. These patients require an MPFL reconstruction, because the hyperlaxity indicates deficiency of the medial soft tissue stabilizers. This surgery is not indicated in an acute knee injury situation and may be performed on an elective basis after the patient has recovered muscle function and conditions are more ideal for surgery.

2 Dislocation with or without underlying anatomic abnormalities, as detailed previously, in which there is a major joint hemarthrosis and MRI evidence of a well-defined or probable patellar or femoral osteochondral fracture. These patients require arthroscopic examination, removal of loose bodies, and fixation of a large osteochondral fracture when present (rare).²⁸ Under ideal circumstances, an MPFL repair is performed and the MRI helps to define the location of the tear. The MPFL may be avulsed at its femoral or patellar attachment, contain an interstitial disruption throughout its substance, or demonstrate a combination of these.^34,^67,⁷⁸ The goal is to restore a functional MPFL to allow immediate knee motion after surgery. The surgeon may elect to perform only the arthroscopic procedure for joint lavage and removal of loose bodies when signs of the acute dislocation (swelling, edema, limitation of joint motion) indicate that only a minimal intervention is required.

3 Dislocation without distinct trochlear or patellar tendon insertion abnormalities in which the primary defect is an acute disruption of the MPFL. In an athletic patient desiring an early return to athletics, an MPFL repair of a well-defined patella or femoral attachment tear may be indicated. Less clear are indications for acute repair of MPFL tears that are interstitial without a patella or femoral attachment tear. These interstitial tears have the ability to heal by conservative treatment and may not require further surgery.^72,^85,⁸⁷

In a majority of chronic knees with recurrent bilateral patellar subluxation or dislocation, patient education is essential to explain the goals of the operative procedure. Patients are advised that any associated articular cartilage damage will likely cause future symptoms that limit athletic participation, even though the recurrent patellar instability has been successfully treated.

CONTRAINDICATIONS

The primary contraindication for extensor mechanism malalignment surgery is the absence of a distinct anatomic defect, because the goal of surgery is to restore normal anatomy of the extensor mechanism. The presence of chronic recurrent anterior knee pain is not an indication for surgery when the specific etiology of the pain symptoms cannot be determined. This point is worthy of emphasis, because many patients present with anterior knee pain in which the specific diagnosis is not apparent despite a thorough and careful examination and evaluation. It is probable that there are pain generation factors related to early patellofemoral cartilage deterioration that is still early in the disease course. Alternatively, peripatellar and intra-articular soft tissues may be an explainable source of pain.^24,³² A common cause of unexplained pain is a subtle infrapatellar neuroma or neuritis of the sensory nerves about the knee joint, discussed in Chapters 42, Knee Pain of Neural Origin, and 43, Diagnosis and Treatment of Complex Regional Pain Syndrome.

Critical Points CONTRAINDICATIONS

• Primary contraindication surgery: absence of a distinct anatomic defect.

• Excessive hip anteversion or abnormal external tibial torsion.

• Distal realignment contraindicated in skeletally immature patients.

• Patellofemoral arthritis relative contraindication; however, in select knees, a realignment procedure is indicated along with a cartilage restoration procedure.

• Severe lower limb muscle atrophy.

• Obesity.

A specific contraindication to extensor mechanism surgery is the presence of excessive hip anteversion or abnormal external tibial torsion as discussed in Chapter 40, Patellofemoral Disorders: Correction of Rotational Malalignment of the Lower Extremity. An MRI or computed tomography (CT) scan for axial rotation of the lower extremity from the hip to the ankle is required. The measurements obtained include femoral anteversion, trochlear groove–tibial tubercle offset, and external tibial torsion as are described. The anterior knee pain is due to excessive lateral patellofemoral forces induced by external tibial torsion or increased femoral anteversion (Fig. 39-1). In these patients, a femoral or tibial derotation osteotomy may be indicated and not a proximal or distal patellofemoral procedure.^58,⁸⁹

FIGURE 39-1 See legend on pg. 999A–C, Demonstration of a classic case with increased femoral anteversion, external tibial torsion, and “squinting” patella requiring femoral rotational osteotomy, possible tibial rotational osteotomy, and not a proximal-distal realignment. A and B, Supine external-internal hip rotation abnormality. C, Standing rotational malalignment shows “squinting patella” and the “miserable malalignment syndrome.” D–J, Physical examination parameters are demonstrated. D, Lateral tibial patellar tendon attachment full extension. E and F, Q-Angle increase with external tibial rotation 30° flexion. G, Q-Angle returns to zero with internal tibial rotation at 30° flexion. H, Normal lateral glide at 30° flexion. I and J, Manual medial glide at 0° and 30° flexion. Palpation of all parapatellar soft tissues, fat pad, patellar tendon, and plica area is also performed.

(A–C, With gratitude from Robert Teitge, M.D.)

A distal realignment is contraindicated in the skeletally immature patient. However, a proximal realignment that may include a QT autograft (for deficient, thin MPFL and retinaculum) may be performed because there is no drilling of osseous tunnels. In cases of recurrent dislocation in the skeletally immature patient, even with an abnormal lateral patellar tendon insertion offset (increased Q-angle), the MPFL reconstruction is usually very successful in providing patellar stability. After growth is complete, the need for correction of the lateral patellar tendon attachment may be assessed, which is frequently not required in the authors’ experience.

A relative contraindication is the presence of patellofemoral arthritis; however, in select knees, a realignment procedure may be performed with a cartilage restoration procedure.^9,^10,^38,^43,^54,^64,^74,⁷⁶

Often, patients with chronic extensor mechanism malalignment disorders have limited their activity, gained excessive body weight, and have articular cartilage damage that limits rehabilitation. These problems cause muscle atrophy of the entire lower extremity. These patients require a comprehensive evaluation and team approach involving nutrition counseling, weight reduction to normal indices, prolonged rehabilitation, occupational rating and modification, and management of patellofemoral pain before any consideration of surgery. An abnormal body mass index in a patient with symptomatic patellofemoral cartilage damage contraindicates extensor mechanism surgery.

BIOMECHANICS OF MEDIAL AND LATERAL PATELLOFEMORAL RESTRAINTS

Three factors influence patellar stability: articular geometry, dynamic muscle actions, and passive soft tissue restraints.^2,²⁷ The geometry of the trochlear groove,² the height and slope of the lateral femoral condyle, and the angle of knee flexion all affect lateral patellar translation. Amis² described objective patellar stability in terms of two factors: the amount of force required to displace the patella a given linear distance from its equilibrium position (translation) and the turning moment required to induce a rotation, such as a lateral tilt. Methods used to determine patellar mobility include manual measurement, instrumented quantitative measurement, and radiographic measurement. Kolowich and coworkers⁴⁷ measured patellar glide by dividing the patella into four quadrants and describing medial and lateral mobility as a fraction of patellar width. Normal subjects had less than two quadrants of translation with the knee flexed 30°. Teitge and associates⁹⁰ developed a stress radiographic method to measure patellar medial-lateral stability. Knees of normal subjects and symptomatic patients were tested between 30° and 40° of flexion with 71 N of force applied. Wide variation was noted in the normal knees, because lateral displacement ranged from 1 to 32 mm, and medial displacement ranged from 2 to 22 mm. The mean difference in lateral displacement between right and left knees was 1.3 ± 1.1 mm, and the mean difference in medial displacement was 1.2 ± 1.08 mm. The investigators determined that a difference between knees in lateral displacement of 3.7 mm, or a difference in medial displacement of 3.46 mm, was abnormal.

Hautamaa and colleagues ³⁹ developed an instrumented measurement device to determine patella medial-lateral translation in the coronal plane. In 17 cadaver knees, with the knee flexed 30 ± 5°, 5 pounds of laterally directed force produced an average of 9.3 ± 0.9 mm of lateral translation. Instrumented measurement of medial-lateral patellar translation was also conducted by Fithian and coworkers²⁹ in normal and symptomatic patients. Right versus left knee comparisons revealed small mean differences in the control subjects for medial and lateral patellar translation (30° flexion, 2.5-lb force, 0.1 ± 1.9 mm and 0.2 ± 1.0 mm, respectively). Significant differences were found between the control and the symptomatic subjects in the mean differences in lateral translation (P < .01), with the patients demonstrating 1.6 ± 2.5 mm difference under a 2.5-pound force at 30° of flexion.

It is well appreciated that the patella is most unstable in the range of 0° to 30° of flexion. When the knee is near full extension, the Q-angle is maximized owing to the external rotation of the tibia. In addition, with the quadriceps muscle relaxed, the patella is not engaged in the trochlear groove and thus is easily mobilized in a medial-lateral direction. As the knee is flexed, patellar stability is increased owing to the combined tensions of the quadriceps muscles and the patellar tendon that pull the patella into the trochlear groove. A shallow or dysplastic trochlear groove allows the patella to displace more easily.⁸¹ A patella alta condition results in a loss of the trochlea geometric restraint until 30° to 40° of flexion when patellotrochlear contact finally occurs.

In regard to passive soft tissue restraints, the MPFL is the primary restraint to lateral patellar translation, providing 53% to 67% restraint up to 30° of flexion.^4,^11,^16,^23,^39,⁷³ Desio and associates²³ examined nine cadaver knees (mean age, 57 yr; range, 43–70 yr) at 20° of flexion and reported that the MPFL provided a mean of 60% of the restraining force to lateral patellar translation. The MPML provided a mean of 13% of the restraining force; the lateral retinaculum, 10%, and the medial retinaculum and medial patellotibial ligament, 3% each. Conlan and colleagues¹⁶ reported similar findings from 25 cadaver specimens (Fig. 39-2); the MPFL provided an average of 53% of the total restraining force, followed by the MPML (22% contribution), the medial retinaculum (11% contribution), and the medial patellotibial ligament (5% contribution). Sectioning of the MPFL decreased the restraining force significantly; the average stiffness decreased from 225 N/cm in the intact specimens to 104.6 N/cm (P = .001).

FIGURE 39-2 Superimposed force-displacement curves, recorded during the 16th testing cycle for one specimen. The medial patellofemoral ligament (MPFL) provided 67% of the medial soft tissue restraint to lateral patellar displacement (13% more than average) and the medial patellomeniscal ligament provided 12% (14% less than average). Curve 1 represents the intact ligaments and curves 2, 3, 4, and 5, the MPFL, medial retinaculum, medial patellotibial ligament, and medial patellomeniscal ligament, respectively, after sectioning.

(From Conlon, T.; Garth, W. P., Jr.; Lemons, J. E.: Evaluation of the medial soft-tissue restraints of the extensor mechanism of the knee. J Bone Joint Surg Am 75:682–693, 1993.)

Nomura and coworkers ⁶⁸ measured the lateral patellar translation in 10 cadaver knees (aged 45–60 yr) with the quadriceps tensed to 10 N and a lateral force of 10 N applied before and after sectioning of the MPFL. Isolated sectioning of the MPFL significantly increased the lateral patellar translation from 20° to 90° of flexion (P < .05).

Amis and associates ³ reported a mean failure load of the MPFL of 208 N from 10 cadaveric specimens (mean age, 70 yr) and interpreted these findings as “surprisingly strong for such an insubstantial appearance.” These investigators also reported that the contribution of the MPFL in restraining lateral patellar translation was greatest with the knee at 0° extension. Significant increases in lateral translation occurred from 0° to 20° after the MPFL was sectioned (P value not provided; Fig. 39-3).

FIGURE 39-3 Graph of patellar lateral displacing force at 10 mm displacement, for the intact knee with 175 N quadriceps tension, and after transection of the MPFL. The drop between the two curves shows that the contribution of the MPFL was greatest in the extended knee. *Significant difference (P value not provided).

(From Amis, A. A.; Firer, P.; Mountney, J.; et al.: Anatomy and biomechanics of the medial patellofemoral ligament. Knee 10:215–220, 2003.)

PREOPERATIVE PLANNING

The physical examination is crucial in preoperative planning for patellar realignment surgery. The examination should be performed with the patient in the standing, sitting, and supine positions (see Fig. 39-1D-J). Palpation of parapatellar soft tissues and the fat pad is performed for swelling and elicitation of pain. The examiner should look for evidence of lateralization of the extensor mechanism and tilt. The patellar compression test should be performed with flexion and extension of the knee to evaluate for articular crepitus or pain. Passive patellar tilt and tightness of the lateral retinaculum should be noted. Patellar subluxation tests (patellar glide at 0° and 30° flexion) should be performed in medial and lateral directions, and patellar mobility noted. Other sources of pain such as neuroma, patellar tendinitis, synovial plica, synovitis, meniscus tears, osteochondritis dissecans, complex regional pain syndrome, and advanced tibiofemoral arthritis should be excluded.

Lower limb rotational alignment, including femoral anteversion and tibial torsion, is measured. The Q-angle is the angle formed between one line connecting the anterior superior iliac spine to the center of the patella and a second line connecting the center of the patella to the tibial tubercle. The Q-angle is measured with the knee in 0° and 30° of flexion. The Q-angle arc is measured at 30° with internal and external tibial rotation. Patients with a physiologic laxity of the posterolateral ligament structures have an increased lateral patellar tendon deviation by virtue of the increased external tibial rotation. The clinical measurement of the Q-angle can be inaccurate for a number of reasons. If the patella is subluxed laterally, the central reference point of measurement is also lateral, with a decrease in the Q-angle measurement unless the patella is carefully positioned within the center of the femoral groove. The Q-angle will vary depending on the amount of knee flexion and foot position; it increases with foot pronation and external tibial rotation. The proximal line to the anterior superior iliac spine is only an approximation.

Conventional radiographic techniques are often inadequate for the assessment of patellofemoral malalignment. The most commonly used Merchant technique requires 30° to 45° of knee flexion. The difficulty with this technique and others is that images are not obtained near full extension where the trochlea is most shallow. As the knee flexes, the trochlear groove deepens and the patella undergoes medialization and becomes more congruent with the femoral sulcus. The extent of trochlear dysplasia and patellar subluxation or tilt may be underestimated on the axial view owing to the amount of flexion required to obtain the image. A lateral radiograph in 30° of flexion may be used to evaluate patellar height, patellar tendon length, and trochlear dysplasia. However, a true lateral radiograph, in which the distal and posterior aspects of the femoral condyles are perfectly superimposed, is required to evaluate trochlear depth, which is measured more accurately on an axial MRI.

CT and MRI have been used to assess lower limb rotational alignment and are important tests for a complete diagnosis of the anatomic abnormalities that may be present. These studies obtain axial images of the patellofemoral joint at or near full extension to evaluate trochlear dysplasia and the tibial tubercle/trochlear groove (TT/TG) distance. Various investigators have studied this distance and found that it is increased in patients with patellofemoral pain and instability.^21,^46,⁸²

Femoral anteversion has been measured by a variety of techniques by many authors. Yoshioka and Cooke ⁹⁷ measured anteversion from the bone in 32 femora and reported a mean of 13.1° ± 8° (range, –11°–+22°) in both males and females when the distal measurement was the tangent across the femoral condyles, and a mean of 7.4° when measured across the epicondyles. These authors conducted a review of the literature on this measurement and found reported normal values were between 8° and 16° in twelve different investigations. Teitge and associates⁹⁰ arbitrarily selected 13° as the goal for correction (see Chapter 40, Patellofemoral Disorders: Correction of Rotational Malalignment of the Lower Extremity).

The technique for measuring tibial torsion by CT has not been standardized and wide variation exists in the literature regarding normal values. Yoshioka and colleagues ⁹⁸ reported a mean lateral tibial torsion of 21° ± 4.9° in males and 27° ± 11.0° in females, a significant difference (P < .05). Eckhoff and coworkers²⁵ reported mean tibial “version” of 37.0° ± 1.7° in control knees and 32.8° ± 1.7° in a group of symptomatic knees. Turner⁹⁴ measured a mean of 19° ± 4.8° in a control group compared with a mean of 24.5° ± 6.3° in a group with unstable patella. Sayli and associates⁷⁹ reported an average of tibial torsion in females of 31.07° for the right side and 30.02° for the left. For males, the averages were 32.7° and 35.26° for the right and left sides, respectively. Tamari and colleagues⁸⁸ conducted a reliability study that examined different methods of measuring tibial torsion. The results showed that clinical methods currently available do not accurately measure true torsion of the femur and tibia. The authors stressed that even so, these methods may be useful for screening and descriptive purposes as indices of true torsion and use of different reference axes could improve their reliability.

MRI has some advantages over CT, including improved ability to image cartilaginous structures, better anatomic approximation of the trochlear articular cartilage groove, and avoidance of radiation exposure. MRI also provides meaningful information about all of the soft tissue structures about the knee joint. The MRI is obtained in standard fashion, with images obtained of the hip parallel to the femoral neck and axial images obtained of the knee and ankle (Fig. 39-4). These individual images are then measured based on the techniques described by Murphy and coworkers⁶⁵ and Guenther and associates³⁷ (Figs. 39-5 and 39-6).

FIGURE 39-4 A–D, An example of a patient’s lower limb rotational magnetic resonance imaging (MRI) profile. A and B, The femoral anteversion for the right knee was 16° on the left and 3° on the right. B and C, The knee version was 3° on the right and 0° on the left. The tibial tubercle/trochlear groove (TT/TG) was 14 mm on the right and 12 mm on the left. C and D, External tibial torsion was 30° on the right and 33° on the left. An increased signal alternation is shown for the lateral patellar facet, which is related to the patient’s primary abnormality of a bilateral external tibial torsion.

FIGURE 39-5 MRI T₁ images are obtained at the hip (A), knee (B), and ankle (C). This allows selection of the proper images for the measurements shown in Figure 39-6.

(A–C, From Parikh, S. N.; Noyes, F. R.; Albright, J.: Proximal and distal extensor mechanism realignment: the surgical technique. Tech Knee Surg 5:27–38, 2006.)

FIGURE 39-6 A, Axial view of the hip is taken through the center of the femoral neck. B, The femoral view is taken at the proximal aspect of the femoral trochlea. C, The proximal tibia view is taken at the proximal insertion of the patellar tendon on the tubercle. The center of the patellar tendon is taken for the TT/TG ratio. D, The axial ankle view is taken at the level of the flat edge of the talar dome (often at the same level and angle as the medial and lateral maleoli).

(A–D, From Parikh, S. N.; Noyes, F. R.; Albright, J.: Proximal and distal extensor mechanism realignment: the surgical technique. Tech Knee Surg 5:27–38, 2006.)

The angle between the femoral neck and the horizontal line is subtracted from the positive angle of the flat portion of the posterior cortex of the distal femur. In rare cases, the rotation of the femoral neck and posterior femoral condyle are in opposite directions, which necessitates addition for the true femoral anteversion. The version of the knee is measured by adding or subtracting the posterior femoral angle (to the horizontal) from the proximal tibial angle (angle between the posterior aspect of the proximal tibia and the horizontal line). The tibial torsion is the difference between the proximal tibia angle and the anterior talus from the horizontal line.

TT/TG is measured using the trochlear groove–patellar tendon insertion distance. TT/TG is measured in millimeters and is the difference from the center of the deepest point of the femoral groove to the center of the patellar tendon attachment to the tibial tubercle. This is done by superimposing the line perpendicular to these points and measuring the distance between the two lines, adjusting for magnification. Dejour and Walch ²² reported a mean TT/TG distance of 12.7 ± 3.4 mm in a group of asymptomatic knees. In a group of knees with patellar instability, the mean TT/TG distance significantly increased to 19.8 ± 1.6 mm (P < .001). The authors concluded that the pathologic threshold is 20 mm, which has been agreed upon by others.⁸⁹ In some knees with trochlear dysplasia, it is difficult to determine the true center point for the measurement, rendering the TT/TG offset to be inaccurate. In these knees, this measurement cannot be used as an evaluation measurement of the desired operative correction.

The TT/TG plays a role in estimating the amount of correction required at the time of tibial tubercle realignment procedure. Intraoperatively, it is easier to accomplish linear corrections (in millimeters) than to assess angular relationships (in degrees), as in the Q-angle measurement. Importantly, this helps to prevent excessive medial translation of the tibial tubercle. The TT/TG measurement may not reflect the lateral deviation of the patellar tendon attachment with knee flexion and external tibial rotation, and therefore, the MRI measurements must be correlated with the physical examination. The tibial tubercle abnormality may also involve both a lateral translation and an outward (external rotation) orientation.