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Investigations

After a careful clinical history and examination, the investigations must include a weight bearing antero-posterior and lateral view of the tibiofemoral joint including the Rosenberg (1) 30° PA view. Many cases of patellofemoral disease are missed because of the failure to take tangential views of the patellofemoral joint at 30° of flexion. This has been well described by Ficat (2) and Merchant (3) and is an essential radiograph to demonstrate the proximal part of the patellofemoral joint.

     
A/P and lateral weight bearing x rays and tangential view at 30° flexion.

Plain radiographs alone will usually demonstrate significant arthritic disease with joint space narrowing, osteophytes and subchondral sclerosis. The joint articulates through at least 130° and significant arthritic damage may be present in part of the arc of motion. The true lateral radiograph will allow assessment as to the presence of any degree of trochlear dysplasia (link with section on Trochlear Dysplasia).  The height of the patella can be measured with a degree of accuracy on this film.  The most useful ratios are those of Caton Dechamps(4) and Blackburn & Peel(5).  The Insall Saslvata(6) ratio can also be used but accurate identification of the bony landmarks are best obtained on CT scans. The use of dynamic CT scanning with views at 0°, 20° and 40° of flexion together with measurement of the height of the patella and tibial tubercle displacement will give a thorough investigation of the simple mechanics and structure of the patellofemoral joint.

Quads relaxed                         Quads contracted
  
Dynamic stacked CT images of the P/F joint at 0° showing subluxation.

The use of MR scanning is now more widespread and this will demonstrate in much more detail lesions of the articular surface on both the patella and the trochlea in its entire extent. Dynamic MR studies are being developed to show the tracking of the patella throughout its early range of movement and these may eventually help to demonstrate specific malalignment syndromes.

Proximal                            Middle                            Distal
   
MR scans showing the upper middle and lower Patella facets in full extension.

The next step in the investigative sequence is the use of arthroscopy. This invasive technique should only be used when non-operative conservative measures have failed to control the symptoms. Useful information can be obtained and recorded about the state of the articular surfaces and the alignment of the patella. It must be remembered, however, that the magnification effect of arthroscopy can make articular lesions look several times larger than they are by direct vision. Treatment by chondrectomy can be performed at the same time to trim any loose unstable flaps of articular cartilage. Arthroscopically controlled extra or intra-articular lateral release can be performed but the place of this procedure remains controversial. In many cases the effects are short lived and it is probably only indicated in a small number of cases, which have early subluxation and tightness of the retinaculum. It is seldom effective on its own especially when there is any significant degree of lateral overload, subluxation or tilt. The development of a lateral peripatellar release may present more logical treatment.


Supero-lateral Portal to view patellofemoral joint.

The arthroscopic assessment should include a thorough assessment of the tibio femoral joint with a view of the patellofemoral joint, the inter-condylar notch and the lower trochlea surfaces from the inferolateral portal. An essential part of the assessment is to view the patella from the top of the supra-patella pouch usually by the supero-lateral portal. This portal can be easily identified by viewing the supra-patella pouch from below. After an incision is made at the highest point and the most proximal point of the pouch, a solid needle is introduced so that the arthroscopic cannula can be rail-roaded over it into the joint without creating a false passage. The joint is then emptied of fluid and careful assessment of the position of the patella throughout the range of movement can be made. Static, or video images are taken of both the lateral gutter and the trochlea groove with the knee at 0°, 20°, 40° and 60° of flexion. Similar assessments are made after realignments procedures to ensure accurate correction of tracking.

0°                                                             20°

Lateral gutter - Patella Groove 0°          -          Lateral gutter - Patella groove 20°
Left knee seen from supero-lateral portal.

References

  1. Rosenberg TD, Paulos LS, Parker RD, Coward DB, Scott SM. The forty-five-degree posteroanterior flexion weight-bearing radiograph of the knee. J Bone Joint Surg Am. 1988 Dec;70(10):1479-83
  2. Laurin CA, Dussault R, Levesque HP. The tangential xray investigation of the patellofemoral joint. Clin Orthop 1979; 144: 16-26.
  3. Merchant AC. Patellofemoral imaging. Clin Orthop 2001; 389: 15-21.
  4. Caton J. Method of measuring the hight of the patella. Acta Orthop Belg 1989; 55; 385-6;
  5. Blackburne JS, Pell TE. A new method of measuring patella hight. J Bone Jt Surg[Br] 1997; 5 9-B: 241-2.
  6. Insall J, Salvati E. Patella position in the normal knee. Radiology 1971;101;101-4.
  7. Yiannakopoulos CK, Mataragas E, Antonogiannatiis. E. The effect of quadriceps contraction during weight-bearing on four patella hight indices. J Bone Joint Surg[Br] 2008:870-3.
 

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