Views: 2 Author: Site Editor Publish Time: 2022-12-22 Origin: Site
Patellofemoral instability (PFI) includes a series of diseases, ranging from mild malaise to obvious dislocation of patella (LPD). LPD is relatively common, with 50 cases in every 100,000 children. The first dislocation usually occurs between 15 and 19 years old. LPD is a debilitating disease, and the dislocation rate after conservative treatment or physical therapy is as high as 70%. Reconstruction of medial patellofemoral ligament is the most widely used surgical treatment. However, as many as 16% of patients have complications, including re-dislocation. In addition, a quarter of patients need follow-up surgery on the other knee joint which is not treated surgically. The long-term risk of progressive cartilage injury and OA after LPD is 6 times higher than that after initial dislocation, which makes many young patients face OA risk in their 30s and 40s. Lack of comprehensive understanding of PFI is one of the main obstacles to restore the consistency of normal patellofemoral joint.
Risk factors of PFI can be divided into two categories: anatomical abnormality and alignment abnormality. The femoral trochlear dysplasia is the most important anatomical abnormality, and the alignment abnormality includes patellar elevation, patellar roll and subluxation. Patellofemoral deformity is caused by biomechanical changes caused by injury of medial stabilizer, increase of Q angle, anteversion of femur and lateralization of patellar tendon insertion. The risk factors of PFI are summarized in Figure 1.
femoral trochlea dysplasia
angle of inclination of transverse pulley
facet asymmetry of pulley
pulley depth
abnormal alignment
high patella
the distance from tibia to trochlear groove (tt-tg) increased
increase q angle
femoral anteversion
The MRI findings of PFI vary with the severity and chronic nature of the disease. Mild PFI cases may be characterized by patellar dyskinesia, which is characterized by edema of the upper and lateral sides of Hoffa fat pad (also known as patellofemoral fat impact). Patellofemoral fat impact is closely related to other risk factors of PFI, including femoral condyle dysplasia, patellar height, increased TT-TG distance, lateral patellar tilt and subluxation. Long-standing patellar dyskinesia leads to cartilage injury and early degeneration of lateral patellofemoral joint.
Acute dislocation of patella (APLD) is the most serious form of PFI. The X-ray plain film shows the discovery of acute injuries, which may include joint effusion, occasional lipid level of fatty arthropathy, fracture of medial patella osteochondral, lateral tilt/subluxation of patella (Figure 8A), and deep lateral sulcus sign caused by impaction injury of lateral femoral condylar cartilage. The specific MRI manifestations of acute LPD include medial stabilizer injury (seen in 96%), lateral patellar tilt or subluxation, osteochondral injury and joint effusion (Figure 2B, C). In most cases, the patella is spontaneously reset after the first dislocation.
Up to 70% of patients will experience recurrent dislocation, and chronic recurrent dislocation may occur. In this case, MRI may show chronic tear of medial stabilizer, medial patellar deformity, ossification of medial patella, patellar-femoral fat impact, cartilage injury and degeneration of lateral patellofemoral joint (Figure 3).
Most acute patellar dislocations are transient and will be spontaneously reset. Sometimes, patients, family members, friends, coaches or trainers will manually reset the patella on the spot. If the patient goes to the emergency department because of patellar dislocation, he will be given conscious sedation. Closed reduction of patella is achieved by gradually stretching the legs. Once reset, clinically check the knee joint for other injuries.
The standard treatment for the first dislocation of patella is non-surgical treatment, and short-term (2-4 weeks) fixation in splint or knee joint fixator can control pain and initial tissue healing after acute attack. During this period, crutches are allowed to bear the weight. After that, patella stabilizing brackets are used for activities, and physical therapy is performed to restore movement, strength and limb control.
Patients usually resume exercise about 3 months after the first attack. Besides, wearing a stent is optional.
In more than 30% of patients, the first patellar dislocation is related to a large amount of knee joint effusion. In this case, it is necessary to perform MRI to identify whether there are osteochondral fractures. The most common location of these fractures is the medial patella or lateral femoral condyle, and surgical treatment is usually recommended in the presence of intra-articular fractures.
During the operation, osteochondral fracture pieces are removed or fixed according to the size of fracture pieces and the quality of cartilage. When the size of osteochondral fracture is ≥ 15 mm, fracture fixation instead of excision is considered. This fixation is performed by an open method using metal screws, bioabsorbable pins or sutures.
In the treatment of fractures, the trend of simultaneous surgical stabilization of patella is achieved by medial repair or MPFL reconstruction. If metal screws are used for fracture fixation, they may have to be removed by other surgical procedures in the future.
There are two schools of thought about the best method of patellar stabilization. The first method is to perform isolated MPFL reconstruction. MPFL is the main constraint factor of lateral subluxation of patella, so its reconstruction will provide the needed stability for patella. MPFL reconstruction is usually performed by quadriceps tendon autograft, hamstring tendon autograft or allograft. The success rate of isolated MPFL reconstruction to restore patellar stability is over 95%, which has nothing to do with the choice of graft. The most common complications of MPFL reconstruction are knee joint stiffness, patellar fracture and recurrent patellar instability.
The second method solves the risk factors of patellar instability, and MPFL reconstruction. In this method, the anatomical risk factors of patellar instability are determined on X-ray film and CT/MRI, including trochlear dysplasia, increased patellar height and TT-TG distance. Once determined, some or all risk factors will be corrected by surgery.
The trochlear dysplasia is solved by trochleoplasty, in which the trochlear groove is deepened (Figure 12A). Trochlear plasty is not very popular in the United States because it involves the invasion of articular cartilage, and theoretically there is a risk of future ischemic necrosis or arthritis.
Patella height or increase of patella height is solved by distal tibial tubercle. In order to increase TT-TG distance, medial or anteromedial tibial tubercle is performed (Figure 12B). Complications of tibial tuberosity osteotomy include nonunion, hardware pain, loss of tuberosity reduction and fracture.
For the tension of lateral retina, lateral retinal release is performed, which shows the increase of patella tilt. The complications of lateral release include persistent swelling and iatrogenic medial instability of patella.
In patients with immature bones, some operations are contraindicated or modified because of the epiphysis.
The femoral attachment point of MFPL is located just below the epiphysis of the distal femur. Therefore, MPFL reconstruction of patients with immature bones should be conducted under strict fluoroscopy guidance to ensure the safe drilling of femoral tunnel.
Distal femur injury can lead to deformity, which may or may not require surgical correction. Similarly, the injury of proximal tibial protrusion can lead to deformity, especially in the medial knee. Therefore, osteotomy of tibial tuberosity is forbidden for patients with open proximal tibial protrusion.
On the contrary, the patellar tendon can be completely or partially displaced medially. When the outer half of the patellar tendon is transferred to the medial side, this operation is called Roux-Goldthwait operation (Figure 12C).
All patients undergoing surgery should be evaluated for patellar instability in the arrangement of coronary limbs and rotating limbs. Increased genu valgus, excessive femoral anteversion and increased external tibial torsion are the risk factors for patellar instability.
For patients with immature bones, guiding growth should be considered when dealing with genu valgus. Epiphyseal screws or tension band plates can span the medial side of the distal end of femoral epiphysis for gradual correction. Osteotomy is needed to correct patients with mature bones for coronary or rotational deformity. The correction indication of genu valgus is > 10 degrees, and the correction indication of rotational dislocation exceeds 20 degrees.
Children (< 10 years old) will encounter complex patterns of patellar instability, which include fixed or habitual patellar dislocation. Several syndromes such as Down syndrome, nail-patellar syndrome, Kabuki syndrome and Rubinstein Taybi syndrome are composed of patellar instability.
It is important to realize that the isolated reconstruction of MPFL is not enough to solve these complicated patterns, because the primary pathology is located laterally, and sometimes the mechanism of quadriceps femoris is shortened, which requires wide lateral release and quadriceps femoris plasty to solve these problems.
In quadriceps femoris plasty, quadriceps femoris mechanism is reoriented and/or prolonged. In the case of neglect or late treatment, these complicated unstable patterns may be encountered later in life.
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