Instabilität der Kniescheibe

A. Arendt, Minesota

The indications for an MPFL reconstruction 

• Not for isolated PF pain, excessive PF lateral tilt &/or translation without instability, or PF arthritis.
• Used to restore the loss of the medial retinacular patella stabilizer due to recurrent lateral patella dislocations.
• MPF laxity must be documentable by physical exam and/or stress radiographs and/or arthrometry testing.

An exam under anesthesia and arthroscopy can be used to document laxity without guarding or apprehension.  Arthroscopy is most helpful to stage cartilage lesions.  Arthroscopy in the face of medial retinacular laxity with the joint distended, typically shows excessive lateral tilt and translation through a passive range of motion, as judged arthroscopically.

The MPFL is used to contain the patella when it is subjected to the extremes of motion secondary to a lateralizing force.  The MPFL is most often used alone without a distal realignment or a trochleoplasty when the bony constructs are normal or near normal.

In my hands, this means that there is:

• Trochlear dysplasia  (T.D.) type A or normal trochlea.
• No trochlear spur.
• A tubero-sulcus angle of 0 to 10° valgus.
• Patella alta < 1.4  (I / S ratio)

When there is patella alta > 1.4, I perform a distal T.T transfer, aiming for a post op measurement of 1.0 – 1.1, based on the same point on the tibia that was used pre-operatively.

I medialize the tibial tubercle when the tubero-sulcus angle is > 10 degrees valgus, or 10mm lateralized.  This is frequently done with a distal transfer at the same time.

Although objective measurements for a tibial tubercle are more exacting, there is not complete agreement in the literature on what this measurement should be and how one should record it.  The most frequented sited work is that by H. DeJour (1994) which sites the threshhold for normality as greater than 20 mm of distance between the tibial tubercle and the center of the trochlear groove on 2 super imposed CT slices.  There is some suggestion in the literature that the normal range is much smaller, with greater than 10 mm as abnormal (Jones R of skeletal radiology, 1995, Beaconsfield, Clinical Orthopaedics, 1994).  One concern of this measurement is that it is a linear measurement and not a ratio, and to my knowledge there has never been a study that has looked at this linear measurement in relationship to the size of the femur.

A greater concern about medialization of the tibial tubercle is over-medialization.  A tibial tubercle that is parallel to the midline of the trochlea at 90° of flexion is normal.  This measurement is easy to visualize clinically, and certainly can be used as an objective measurement inter-operatively in regards to how medial one should place the tibial tubercle when excessive lateralization is felt to be present.  One concern in regards to the current wave of AMZ, is that there are few guidelines to help the surgeon judge the amount of medialization of the tibial tubercle intra-operative, despite an objective pre-operative measurement (ie) a TT-TG.

Visual inspection of the knee is unreliable in determining the TT-TG measurements compared to CT measurements (Shakespeare, 2005).

If a supratrochlear spur is present with T.D. type A, I remove it alone.

If T.D. is present to the degree that it prevents smooth engagement of the patella into the groove, a trochlear-plasty is performed.

In my patient population over the last 4 years, this has occurred 3 times (< 5% of surgical cases). 

Anatomy and Biomechanics of Patellofemoral Restraints (with particular reference to surgical concerns)

Medial Side

The Medial PF Ligament (MPFL) attaches to the femur 10 mm proximal and 2 mm posterior to the medial epicondyle, in the saddle between the medial epicondyle and the adductor tubercle.(Fig. 1).  Its patella attachment is approximated at the junction of the upper and middle thirds of the patella, typically at the location where the perimeter of the patella becomes more vertical.

It is the prime soft tissue restraint to lateral patella displacement.  However, it is only significant in early flexion.

As the knee progresses in flexion, trochlear geometry, patellofemoral congruence, and in particular the slope angle of the lateral wall of the trochlea provide the major restraints to lateral patella displacement (Farahmand et.al., 1998).   In trochlear dysplasia, the groove is often not only flattened, but shortened.  The shortened groove combined with a high riding patella (patella alta) will create a larger arc of motion before the patella is protected by the confines of the lateral trochlear wall.

How strong is the MPFL? (Table 1)

MPLF Elongation to failure: mean 25.8 mm, standard deviation 6.6, range 14 to 34 (Amis et.al., 2003).   Graft stiffness (change in length / ultimate load) likely matters.

MPFL attachment sites – isometric / isoanatomic

There is no evidence to date that the MPFL functions isometrically.  MPFL is most loaded (longest) in full extension with quads loaded.

With quads relaxed, the “longest” length of the MPFL thru ROM is debated.

By 60°, morphology of the patellofemoral joint contains the patella in normal knees. (Figs. 2 & 3).

Surgical implications

The ideal graft would have similar stiffness, but be stronger, than the native MPFL.  The current tissue used to reconstruct the MPFL is significantly stiffer than the native MPFL.

MPFL reconstruction with “stiff grafts” can produce large increases in PFJ loading if small errors in graft length and/or attachment site are present.  This will have its biggest consequence if the graft length is “too short” for its arc of motion, and the length change thru an arc of motion is restricted.  This will result in reduced ROM, increased forces on the medial patella facet, or both.

Graft attachment points: More research is needed.  Some agreement that length change pattern depends principally on the femoral attachment point.   The least change was with a point more distal on the patella and more proximal on the femur (Steensen, 2004).  This was also the site that had the longest length between the 2 points.  For one cadaver study (Smirk, 2003), the femoral attachment site was most sensitive to position change, especially superior and anterior.  The ligament was “longest” at 60° of flexion.

The graft length should allow the patella to enter the trochlear from a lateralized position, as dictated by normal PF kinematics, and allow the slope of the lateral trochlear wall and the lateral patella facet to engage its trochlear position gradually.

Intra-operatively, one should adjust the attachment sites to minimize the length change with knee flexion.  If lengthening occurs in flexion, one can move the femoral attachment site more distal. If lengthening occurs in extension, one can move the femoral attachment site more proximal.

We still lack any objective evidence for an MPFL graft tensioning protocol.  This must be a compromise between over-constraint causing medial patella pressure vs. slackness which allows patella subluxation in early flexion. It appears prudent to tension your graft with the knee contained in the groove at the ROM where your graft length in the longest.

References:

1. Amis AA, Firer P, Mountney J et al.  Anatomy and biomechanics of the medial patellofemoral ligament.  Knee 10:215-220, 2003.
2. Arendt EA:  Patellofemoral and patellotibial ligaments: Anatomy and Biomechanics, paper # 134, ISAKOS, Florence, Italy, May 2007.
3. Beaconsfield T, Pintore E, Maffulli N, Petri GJ.  Radiological measurements in patellofemoral disorders: a review.  Clin Orthop. 308:18-28, 1994.
4. Dejour, H., G. Walch, et al. "Factors of patellar instability: an anatomic radiographic study." Knee Surg Sports Traumatol Arthrosc 2(1): 19-26, 1994.
5. Desio S M, Burks RT, Bachus KN.  Soft tissue Restraints to lateral patellar translation in the human knee. Am J Sports Med. 26(1):59–65, 1998.
6. Elias J J, Cosgarea A J.  Technical errors during medial patellofemoral ligament reconstruction could overload the medial patellofemoral cartilage: a computational analysis.  Am J Sports Med 34(9):1478–85, 2006.
7. Farahmand F, Senavongse W, Amis AA.  Qualitative study of the quadriceps muscles and trochlear groove geometry related to the patellofemoral joint.  J Orthop Res. 16:136-43, 1998.
8. Jones R. CT determination of tibial tubercle lateralization in patients presenting with anterior knee pain. Skeletal Radiology 24(7):505-9, 1995.
9. Mountney J, Senavongse W, et al.  Tensile strength of the medial patellofemoral ligament before and after repair or reconstruction.  J Bone Joint Surg 87B(1):36-40, 2005
10. Powers CM, Landel R, Perry J.  Timing and intensity of vastus muscle activity during functional activities in subjects with and without patellofemoral pain.  Phys Ther. 76(9):946-55; discussion 956-67, 1996.
11. Shakespeare D, Fick D.  Patellar instability: can the TT-TG distance be measured clinically?  Knee 12:201-204, 2005.
12. Smirk C, Morris H.  The anatomy and reconstruction of the medial patello-femoral ligament.  The Knee 10:221-227, 2003.
13. Steensen R N, Dopirak RM, McDonald WG 3rd.  The anatomy and isometry of the medial patellofemoral ligament.  Am J Sports Med. 32(6):1509–1513, 2004.