Functional Anatomy and Biomechanics
- Normal trochlea is concave with strict correlation between the bony contour and overlying cartilage
- The retro-patellar articular surface - larger lateral facet,
the median ridge and medial facet. Fig. 2
- There is the non-articulating “odd” facet,
which is located medial to the medial facet
- 4 types of trochlear morphology which can predispose to patellar instability described by Dejour 1 Fig. 3
- Patellar dysplasia can occur.
Wiberg described 3 patellar shapes: V shape,
L shape (vertical,
small medial facet),
and a flat patella 2 Fig. 6
- Patellar dysplasia can lead to abnormal point loading at the lateral facet and cause anterior knee pain.
Stability
Passive and active stabilisers
- Active
- Quadriceps muscles
- In particular Vastus medialis obliquus (VMO)
- Deficient/ relaxed VMO reduces lateral patellar stability by 30% at 20 degrees of knee flexion 3
- Passive
- Bony congruence between grooved trochlea and ridged patella
- Medial patellofemoral ligament,
retinaculum
- MPFL deficient knee,
force required to displace the patella laterally is reduced by 50% with knee in extension 4
Biomechanics
- The patella provides a biomechanical advantage for the knee extensor mechanism
- Magnifies the forces generated by the quadriceps muscle.
- PFJ therefore subjected to high joint reaction forces – 7x times body weight during deep squatting 5
- Patella reduced and initially engaged into the trochlear groove at 10-20 degrees of flexion
- Mean contact area of the PFJ increased from 126 mm2 to 560 mm2 from full extension to 45 degrees of flexion 3
Risk factors for instability - Imaging assessment
Radiographs
- AP,
Lateral (weight bearing at 30 degrees flexion),
Axial (Merchant) views Fig. 7
- Trochlear dysplasia (TD) can be assessed on the lateral view,
characterised by several signs 1: Fig. 4
-Crossing sign
-Double contour
-Supra-trochlear spur or nipple
- Patella height on lateral radiograph.
2 methods include: Fig. 8
-Insall-Salvati index or ratio (ISI) - Normal range 0.8-1.2
-Caton-Deschamps index (CDI) - Normal range 0.6-1.3
-Angle formed by the quadriceps and patella tendon in the frontal plane.
-Often measured on long leg views.
For males,
average Q angle is 10 degrees and for females 15 degrees.
CT/MRI
- Sagittal patella morphology can lead to inaccurate assessment of patella height 6.
- In conjunction with usual ISI,
a modified ISI can be used to calculate patella height Fig. 11
-3 variations in shape of the patella with regards to the length of the articular surface and lower pole have been described by Grelsamer. Fig. 10
- Tibial tubercle-trochlear groove distance (TT-TG) Fig. 12
-Assesses lateralisation of the tibial tubercle and is thought to be an accurate assessment of patellar instability
-Relies on two axial cuts,
the first through the deepest point of the trochlear groove and the second through the proximal part of the tibial tubercle where the patella tendon inserts
-These two cuts are projected on a line tangential to the posterior femoral condyles and the distance between the two points represents the TT-TG distance
-A value of over 20 mm is considered abnormal
-Measurements derived from CT are not interchangeable with MRI,
the latter having been shown to underestimate TT-TG values often by up to 4 mm 7.
– good correlation with x-ray
-ISI on MRI is different – Patella alta > 1.5.
Patella baja < 0.74 8.
-Clinically,
maltracking can be visualised as the “J-sign” which is represented by excessive lateral patellar excursion at the termination of extension
-A technique described utilises an inflated plastic ball which was permitted to deflate as the patient extended their knees 9.
-During knee extension,
a series of fast gradient-echo sequences were performed.
Fig. 14
Surgical Options
Lateral release and medial reefing
-Release of the lateral patellar retinaculum is a purely soft tissue procedure
-There is evidence to support the use of lateral release in cases of excessive lateral pressure syndrome (ELPS) Fig. 24 ,
a cause of anterior knee pain due to tightness and hypertrophy of the lateral retinaculum 10
-Medial reefing (medial capsular placation) describes a procedure of tightening the medial capsulo-ligamentous structures with suture material
-The medial retinaculum contributes just 13% to the medial restraint force of the patella,
versus over 50% for the MPFL
-Best seen on MRI
-excess scar tissue formation at the site of surgery,
or fluid herniating through the defect in the lateral retinaculum
-medial patellar instability
-over-tightening during medial reefing can lead to abnormal loading of the medial PFJ and subsequent pain
MPFL reconstruction
-used to treat lateral patellar instability due to damage and excess laxity of the medial retinacular structures and MPFL
-indicated in patients with at least 2 documented PFJ dislocations and excessive lateral patellar mobility
-gracilis or semitendinosus tendon graft
-not performed for patellofemoral pain
-PFJ osteoarthritis is a relative contraindication
-Femoral and patella tunnels seen Fig. 15
-accurate anatomical placement of the femoral tunnel is critical and its location can be pinpointed on a true lateral radiograph of the knee 11 Fig. 16
-Post-operatively,
the graft should remain taught and demonstrate low T1 and T2 signal intensity
-Over tightening of the graft can lead to overload of the medial patellar facet and subsequent osteoarthritis
-Non-anatomical position of the graft can result in recurrent lateral laxity
-Mal-positioning of the patellar and femoral tunnels can lead to disabling symptoms often necessitating revision surgery
-Patellar fracture is a rare but recognised complication through the patellar tunnel.
Distal realignment procedures Fig. 19
-Distal realignment procedures are considered effective in correcting patellar maltracking and offloading the PFJ in patients with an increased TT-TG offset.
-The two main surgical techniques performed are the Elmslie-Trillat procedure and the Fulkerson osteotomy
-Elmslie-Trillat procedure combined soft tissue and bony
-medial TT transfer hinged on a distal periosteal attachment
-lateral release and medial reefing
-The Fulkerson osteotomy is a modification of the Elmslie-Trillat
-anterior as well as medial translation of the tibial tubercle.
-The Marquet procedure (historical technique)
-anterior translation (elevation) of the tibial tubercle by 2-2.5cm
- cortical bone graft often interposed between TT and donor site
-Disadvantages include a high incidence of skin necrosis
-does not address the Q angle
-Hauser procedure (historical technique)
-medialised the TT therefore shifting the tubercle posteriorly
-effective in preventing lateral patellar dislocations
-predisposed to early PFJ osteoarthritis
-Radiographs will show prominence of the TT with often two parallel screws running in an anterior posterior direction
-Assessment of bony union between the TT and the anterior tibia should be made
-Complications include over-correction with excess medialisation of the TT leading to pain in the medial PFJ
-non-union or fragmentation of the TT osteotomy
Trochleoplasty
-Open trochleoplasty is indicated in patients with symptomatic patellofemoral instability in the presence of TD
-aim is to reconstruct the trochlear groove,
thereby stabilising the patella during the initial 30 degrees of flexion
-achieved by elevation of the lateral facet,
deepening of the sulcus and thus restoration of “normal” anatomy.
-Trochleoplasty considered a demanding procedure and lacks familiarity among many knee surgeons.
-Reduction of the trochlea bump and deepening of the trochlear groove
-PFJ congruency
-Screws are often used to fix the osteotomised trochlear in place
-Due to its high surgical demands,
on-going patellofemoral incongruence remains a possibility.
-Concerns of viability of the articular cartilage and subchondral bone necrosis
-Proud intra-articular screw – chondral wear
Roux-Goldthwait Procedure (historical technique) Fig. 22
-The patella tendon is split vertically in half
-the lateral half detached from the TT and pulled under (deep to) the medial half and reattached to the anterior tibia.
-This pulls the patella medially and helps prevent excess lateral displacement during flexion.
-These are best appreciated on MRI with crossing over of a vertically split patella tendon
-Increased incidence of osteoarthritis and recurrent dislocation when compared with a group that underwent medial ligament reconstruction 12