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Patellar alta: More precise measurements are needed for operative management

Patellofemoral Update

Patella alta has been a known associated factor of recurrent lateral patella dislocations for decades. The Lyon group recognized patella alta as and one of four main “objective” patella instability risk factors, and suggested distalization of the tibial tubercle as a surgical solution for the problem.

The biomechanical result of patella alta is decreased bony constraint secondary to the delayed entry of the patella into the trochlear groove, thus there is a greater arc of knee motion in early flexion in which the patella is potentially subjected to lateral forces leading to a patella dislocation. This delayed entry into the groove is magnified in the face of trochlear dysplasia when there is a shortened or shallower groove as well. This delayed entry also alters the patellofemoral contact area, which in conjunction with known forces at early flexion angles, results in an increase in patellofemoral (PF) stress. This increased stress could lead to pain and chondral wear.

With the historical backdrop of patellar infra contraction syndrome (PICS), many U.S. surgeons have been hesitant to treat patellar alta. Surgeons now realize that optimizing patellar height can be performed without causing patella infra. However, distalization of the tuberosity with poor surgical technique or wrong indications can lead to disastrous results.

Operative management

Although patella alta can be a physical examination sign, more precise measurements are needed for operative management. At first glance, it would appear the diagnosis is direct. That is, the patellar is higher than normal patellar height ratios (0.8 to 1.2), such as Insall-Salvati, Blackburn-Peel or Caton-Deschamps. Most surgeons who perform distalizations advocate using a ratio that is independent of the tuberosity (such as Insall-Salvati), as the ratio will not change after moving the tuberosity. Thus, the Caton-Deschamps ratio is commonly used preoperatively, as this objective measure can be used postoperatively as well.

Traditional measurements were calculated on a sagittal radiograph of the knee. Although these measurements are independent of knee flexion, convention has it that they are usually taken in some degree of knee flexion. It would seem that preoperative planning would merely involve obtaining a true lateral, correcting for magnification and then subtracting the patellar subchondral bone length from the distance of the patella to the tibial reference. In many cases, this is true. However, to avoid misdiagnosis/mistreat of the outlier patient, it is necessary to engage in a more thorough evaluation.

Advantages and disadvantages

With the widespread use of slice imaging, most typically MRI, another variable has complicated the measurement of patella height. Although we can use the traditional methods described for sagittal radiographs, advantages and disadvantages become evident. Measurements on MRI have the advantage of:

• Seeing the engagement of the patella cartilage with the trochlea grove; and
• MRI shows chondral surface, which is often different that osseous surface. Staeubli showed that plain films may under or overestimate the area of chondral coverage.

Measurements on MRI have the disadvantage of:

• The best cut of patella and the tibial tubercle are often not on the same slice;
• The degree of quadriceps contraction is difficult to quantify and will change the patella height when the knee is in extension;
• MRI and CT measurements of patella height are not equal (likely due to taking different points of measurement due to clarity of soft tissue structures on MRI), and the confidence intervals of “normal” height cannot be used interchangeably; and
• When the MRI is taken after an acute dislocation with a large effusion present, significant patella tilt is often present which can create a “false” image of true patella position.

So where does this leave us. We know patella alta is one of the risk factors associated with recurrent patellar lateral instability (dislocations/subluxations) noting that other factors include: trochlear dysplasia (a shortened and flattened trochlea), excessive lateral position of the tibial tuberosity and patholaxity of the medial PF ligament.  In more dysplastic cases, it may not be that the patella is too high, but rather the trochlea may be too short.

• Below are steps to analyze the problems and conditions that can complicate surgical planning:
• Excessive patellar lateral positioning (ie, increased lateral tilt and lateral subluxation);
• Pathologic short trochlea; and
• Abnormal patella shape, especially in the “nose” region of the patella.

Patellar height

If measured radiographically, the excessive lateral patellar tilt and translations may result in a false measurement of patellar height as a portion of the patellar length has the optical/radiographic illusion of shortening. That is, a portion of the length is decreased radiographically similar to a windshield wiper movement (for example, the windshield wiper arm (patellar tendon) is a constant length, yet the height of the wiper blade (patella) is closer to the base of the windshield at extremes of its arc). In these cases, measurement of the true length of the patellar tendon requires an image with the patella in front of the trochlea. This can be a flexed knee image or repeat height evaluation intra-operatively after the patella is repositioned to the central aspect of the trochlear groove.

The patellotrochlear index as described by Biedert and Albrecht measures patellar position by direct assessment of the patella and trochlear chondral articulation on a sagittal MRI cut. Normally, contact between the articular surface of the trochlea and the articular cartilage behind the patella is about one third of the length of the patellar cartilage (measured using the patellotrochlear index). Additionally, the ratio of the patella articular surface to the physeal scar (patellophyseal index is the height of patella above the anterior physeal line divided by length of patellar cartilage).

Another tool to evaluate the trochlear length was described by Biedert: the lateral condylar index. This ratio compares the proximal extent of the articular cartilage of the lateral condyle posteriorly to the proximal extent of the trochlea. A lateral condyle index less than or equal to 90 suggest the trochlea is too short. Treatment is directed at the pathology for these patients. That is, the trochlea may be lengthened as detailed by Biedert in Patellofemoral Pain, Instability, and Arthritis.

Tuberosity distalization

Distalization of the patella may be effected by either tuberosity distalization as per Neyret in patients with closed physes or, less frequently, shortening of the patellar tendon in patients with open physes as per Andrish. This normalization of the patellar position does not lead to PICS unless other factors are involved, for example, excessive scar tissue production.

 

Radiographs show patellar infra and patellar chondrosis after a patella distalization.

Source: Fulkerson JP

It is necessary to appreciate that patella distalization is less straightforward than it appears on a plastic knee model. The distalization requires medial and lateral releases of the capsule at the patellar tendon margins with unknown consequences to blood supply. It also may lead to altered stress in the extensor mechanism leading to difficulties in regaining full flexion. Non-unions, delayed unions, tibial fractures, tibial tubercle fractures, are potential postoperative surgical complications in addition to patella infera. Patellar infra (not infrapatellar contracture syndrome) and patellar chondrosis are evident following a patella distalization as seen in the above radiographs.

Setting goals

When considering indications, the first goal is to be sure the patella is actually dislocating laterally, which should be evaluated by history and physical exam. While case series demonstrate efficacy of distalization in the hands of Neyret, the role distalization plays in the treatment of patellar instability still is evolving. A study comparing medial patellofemoral ligament (MPFL) reconstruction alone vs MPFL reconstruction in conjunction with distalization will be important.

Currently, one population that the two procedures may be combined using a biomechanical rationale is the case of extreme alta. In these patients, the MPFL length changes in extension may make it extremely difficult to arrive at a femoral attachment site that allows the desired MPFL attachments site distance changes in early and deep flexion. In these cases, distalization would be performed prior to the MPFL reconstruction.

Many surgeons ask us about what index we use and how we measure alta. The answer is there is not one right measurement scheme to use. You must measure patella height and patella engagement, and come up with a surgical plan that best addresses the surgical goals. Too many times we see tibial tubercle distalization performed without a single recorded measurement in either the clinical preoperative note or in the operative note. It is important to not only perform the surgery correctly, but also perform it only when indicated.

References:

Ali SA. AJR Am J Roentgenol. 2009; doi:10.2214/AJR.09.2729.

Andrish J. Surgical options for patellar stabilization in the skeletally immature patient. Sports Med Arthrosc. 2007;15(2):82-88.

Biedert RM. The patellotrochlear index: a new index for assessing patellar height. Knee Surg Sports Traumatol Arthrosc. 2006;14(8):707-712.

Biedert RM. Trochlear lengthening osteotomy with or without elevation of the lateral trochlear facet. In: Zaffagnini S, Dejour D, Arendt EA, ed. Patellofemoral Pain, Instability, and Arthritis. Berlin: Springer; 2010:344.

Staeubli HU. Magnetic resonance imaging for articular cartilage: cartilage-bone mismatch. Clin Sports Med. 2002;21(3):417-433.

Upadhyay N. Effect of patellar tendon shortening on tracking of the patella. Am J Sports Med. 2005;33(10):1565-1574.

Ward SR. The influence of patella alta on patellofemoral joint stress during normal and fast walking. Clin Biomech. 2004;19(10):1040-1047.

• Jack Farr, MD, is medical director, OrthoIndy Cartilage Restoration Center of Indiana, and director, OrthoIndy Sports Medicine Fellowship, Indiana Orthopedic Hospital, Indianapolis, Ind. He is also professor of Orthopedic Surgery, Volunteer, Indiana University Medical School. He can be reached at 1260 Innovation Pkwy., #100, Greenwood, IN 46143; email:indyknee@hotmail.com.
  Elizabeth A. Arendt, MD, is professor of Orthopedic Surgery at University of Minnesota. She can be reached at University of Minnesota, Department of Orthopaedics, Box 492, 420 Delaware St. SE, Minneapolis, MN 55455-0374; email: arend001@tc.umn.edu.
• Disclosures: Farr receives research or institutional support from Sanofi Company formally Genzyme Biosurgery, Histogenics, Johnson and Johnson Companies, Depuy/Mitek, RTI Biologics and Zimmer; receives miscellaneous non-income support, commercially derived honoraria or other nonresearch-related funding from, Arthrex, Sanofi Company formally Genzyme Biosurgery, Johnson and Johnson Companies, RTI Biologics and Zimmer; receives royalties from Arthrex, Johnson and Johnson Companies, Depuy/Mitek, Depuy Orthopaedics; and is a consultant for Arthrex, Sanofi Company formally Genzyme Biosurgery, Johnson and Johnson Companies, ISTO Technologies, RTI Biologics Cartilage Advisory Panel, and Zimmer. Arendt is a consultant for Tornier.