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MPFL reconstruction addresses patellar instability in skeletally immature patients
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Patellar instability is more common in children younger than the age of 16 years with an incidence of approximately 43 per 100,000 children per year. Although more than 100 different surgical techniques for patellar stabilization have been described in the literature, medial patellofemoral ligament reconstruction has been increasingly accepted as the treatment of choice for most patients with recurrent patellar instability.
When medial patellofemoral ligament (MPFL) reconstruction is performed in children with open distal femoral physis, some technical modifications need to be considered to avoid potential complications. The following Surgical Technique article describes the surgical considerations and our preferred surgical technique for MPFL reconstruction to address patellar instability in skeletally immature patients, which comprises of a single patellar tunnel, gracilis autograft and femoral tunnel interference fixation (Figure 1).
Images: Kakazu R, University of Cincinnati
Surgical considerations
There are several types of patellar instability, of which the most common is lateral patellar instability. Type 1 (first-time patellar dislocation) is typically treated conservatively. Type 2 (recurrent patellar instability) and some type 3 (dislocatable patella) could be treated by MPFL reconstruction. Type 4 (dislocated patella) may require more complex surgical treatment, such as quadricepsplasty. Like any surgical procedure for a complex issue like patellar instability, clinical and imaging findings, medical comorbidities, compliance, social situation and conservative treatment options have to be considered before making the decision for surgical treatment.
There are several graft choices for MPFL reconstruction, including gracilis tendon, semitendinosus tendon, quadriceps tendon or allograft tissue. Studies have not shown any significant differences in outcomes related to the choice of graft because all these grafts are stiffer than the native MPFL. Our graft choice is gracilis autograft due to its smaller diameter (3.5 mm) requiring a smaller patellar tunnel and reluctance to use allograft tissue in younger patients. The length of the gracilis tendon is around 20 cm and has not been an issue when used in a single patellar tunnel. Our second choice of graft is the semitendinosus tendon.
For patellar and femoral-sided fixation, there are several choices, including bone tunnels, suture anchors, interference screws or soft-tissue fixation. Though soft-tissue fixation can alleviate safety concerns, like femoral drilling near distal femoral physis, there is a potential for stretching of soft tissues with time and with growth, especially if the soft tissues are incompetent to begin with. During MPFL reconstruction in skeletally immature patients, the femoral attachment of graft should be placed below the level of distal femoral physis.
Surgical technique
The patient is placed supine on a radiolucent table (Figure 2). General anesthesia and femoral/sciatic nerve block is performed. A well-padded, non-sterile tourniquet is applied to the affected thigh. Formal time-out is performed, which is followed by thorough examination under anesthesia for both lower extremities to assess range of motion, patellar mobility and patellar tracking (Figure 2.
Preoperative antibiotics are administered, and the lower extremity is prepped and draped in the standard fashion. The tourniquet is then inflated after exsanguination of the lower extremity.
Knee arthroscopy
Knee arthroscopy is performed using standard portals. Besides the patellofemoral joint, the meniscus, ligaments and tibiofemoral joint are evaluated.
For the patellofemoral joint, the position of the patella, lateral overhang and patellar tracking with knee flexion and extension are assessed. The chondral surfaces of patella and trochlea are probed, and any cartilage lesions are documented. If any loose cartilage flaps are present, these are then debrided.
Once the trochlear morphology is assessed, the trochlear bump from anterior femoral surface can be measured. In patients with normal preoperative MRI, one can choose to avoid knee arthroscopy, though we routinely perform it before and after MPFL reconstruction.
Hamstring harvest
During hamstring harvest, a 2.5-cm transverse skin incision is placed on the posteromedial aspect of popliteal crease over the palpable gracilis tendon (Figure 3). A leg holder is not used during surgery as the figure of 4-position of the leg helps facilitate tendon palpation and exposure. The tendon is delivered out of the incision using a right-angle clamp and is freed of its attachments (Figure 4). The tendon is harvested using an open-tendon harvester proximally to detach the tendon from its musculotendinous junction, and a closed-tendon harvester is used distally to detach the tendon from its tibial attachment. The tendon is cleaned on the back table off its muscle attachments and then secured in a saline-soaked sponge. No suturing of the tendon is performed at this time.
Alternatively, the gracilis tendon can be harvested using a traditional anteromedial approach.
Patellar tunnel, femoral tunnel
A 4-cm incision is made along the medial border of the patella in its superior two third. Dissection is performed down to patella. Electrocautery is used to perform subperiosteal dissection starting about 5 mm medial to the medial border of the patella and proceeding medially. Transverse fibers of medial retinaculum are identified as these insert on the patella. Dissection is performed using Metzenbaum scissors to cut these fibers and identify the plane between layer 2 (retinaculum) and layer 3 (capsule).
Typically, this plane is developed above the widest part (equator) of the patella where the future patellar tunnel would be created. Care is taken to avoid capsular penetration and to prevent inadvertent intra-articular placement of graft. Once the plane is identified, blunt dissection is performed in this plane toward the medial epicondyle and adductor tubercle.
For patellar tunnel preparation, two 1-cm drill holes are made using a 3.5-mm drill bit: first from medial to lateral and second from anterior to posterior that theses connect to create a single tunnel under a bone bridge (Figure 5). After the first drill hole is made from medial border of patella, an outflow arthroscopic cannula or similar smaller instrument is inserted in it to mark its position and facilitate the drilling of the second drill hole. Curved curette is used to chamfer the tunnel and remove bone debris. An OS-6 half-circle needle on a Vicryl suture (Ethicon) is passed through the tunnel.
A loop is created at the end of the suture, and the musculotendinous thin end of the gracilis tendon is placed in the loop and pulled through the patellar tunnel. Both tendon ends are then whip stitched with a #2 Fiberwire (Arthrex) suture and sized for the typical 5-mm to 6-mm diameter of doubled graft. The graft is wrapped in saline-soaked sponge.
The femoral tunnel is prepared with fluoroscopic assistance. The knee is flexed over a triangular bump and a perfect lateral view of the distal femur is achieved so the distal and posterior femoral condyles overlap. A beath pin is placed at the femoral attachment point according to the described radiographic landmarks. In the skeletally immature patient, the pin is placed at the level of the physis on the lateral fluoroscopic view and advanced about 1 cm using a mallet.
Due to the undulating nature of the physis, when the pin is placed at the level of physis on lateral view, it is below the physis, which is confirmed on anteroposterior view (Figure 6). If needed, the femoral attachment point can be further verified by making a longitudinal incision around the pin and palpating the bony landmarks or by visualization of adductor tendon and/or superficial medial collateral ligament. An isometric assessment can be performed at this time by wrapping the graft around the pin and putting the knee through a range of motion to check for graft length changes. If needed, the pin can be repositioned at this time, though in Parikh’s experience, it is seldom necessary. The beath pin is then advanced under fluoroscopic guidance, from medial to lateral, posterior to anterior (to avoid the intercondylar notch) and parallel to the physis (to exit from the lateral side). The pin is then over-reamed with appropriate size reamer as per the size of the doubled graft and to the depth as determined by the graft length. Once the femoral tunnel is prepared, a nitinol guidewire for interference screw is inserted in the drilled tunnel alongside the beath pin (Figure 7).
Graft passage, fixation
A Kelly clamp is placed from the anterior incision between the two layers on the medial side where the previous dissection was performed. The clamp is penetrated through the retinaculum and out of the medial incision next to the beath pin at the femoral attachment point. The clamp is used to pull the suture loop from medial to anterior incision. The graft sutures and doubled graft are shuttled out of the medial incision using the suture loop. The suture ends are placed in the eyelet of the beath pin, and the pin is pulled out from the lateral side. As the sutures are pulled from the lateral side, the doubled graft is pulled into the tunnel.
It is important to verify the passage of the graft in the femoral tunnel and that it does not get bundled up at the tunnel entrance. One of the most important technical parts of surgery is graft tensioning. Ideally, the graft should not be tensioned to pull the patella medially, but it should rather work as a passive restraint to prevent patella from subluxing laterally. To achieve this, once the graft is passed, the knee is placed through a range of motion. Then, with the knee in about 45° to 60° flexion and with the patella engaged in the trochlea, a clamp is placed on the exiting sutures against the skin on the lateral side to achieve tentative fixation of the graft. The knee is then extended and patellar mobility is confirmed to be about one quadrant on both medial and lateral sides.
Excessive tightening can lead to medial overload. Once adequate position of the graft is confirmed, a Matryx interference screw (ConMed), typically 5.5 mm x 25 mm, is inserted over the previously placed nitinol guidewire. (Figure 8). The size of the screw is typically the same as the tunnel size. The complete insertion of the screw is confirmed by fluoroscopic image of the screw driver as the screw itself is radiolucent. Hardware prominence can be symptomatic, and complete insertion of the screw is ensured.
Final assessment
The arthroscope is re-inserted to assess the placement of the MPFL graft and patellar tracking. In our experience, the graft was inadvertently placed intra-articularly on two occasions and was revised intraoperatively: during a revision case with increased scar tissue on the medial side; and in a patient with Turner syndrome when a capsular rent was not identified during plane creation and graft was passed through the rent. Thus, if the capsule/third layer is penetrated during initial dissection and plane creation, it is better to close the capsule with sutures and then proceed.
Once the extra-articular placement of the graft is verified, the adequate functioning of the graft can be confirmed through the arthroscope. Since the MPFL functions more during early knee flexion (0° to 30°), the graft should be taut during this early range of motion. As the knee is further flexed and the patella is engaged in the trochlea, the graft loses its tension and appears to be relaxed. Finally, the tracking can be assessed. If needed, limited debridement along the medial border of patella can be performed.
The deep closure is performed with the knee flexed over a triangular bump to prevent overtightening of medial structures. The medial retinaculum is imbricated in a pants-over-vest fashion using absorbable sutures. Remainder of the closure and other incisions and portals are closed in a standard fashion. Well-padded dressing is applied, followed by a CryoCuff (AirCast) and knee immobilizer.
Postoperative course
Physical therapy is started in 3 days to 4 days once initial postoperative pain is controlled. Patient is allowed to bear weight as tolerated in a knee immobilizer using crutches for the first 3 weeks to 4 weeks. There is no restriction in range of motion, and the goal is to obtain 90° knee flexion by 3 weeks and 120° by 6 weeks.
It is important to stress on quadriceps strengthening during follow-up visits. Strength and functional gains are assessed using hop tests and isokinetic dynamometer (Biodex) testing. Patients are released to full activities, including sports, once they clear all functional strength tests, which is typically around 6 months.
- References:
- Parikh SN, et al. Am J Sports Med. 2013;doi:10.1177/0363546513482085.
- Parikh SN, et al. Orthop Clin North Am. 2016;doi:10.1016/j.ocl.2015.08.016.
- Parikh SN. Tech Knee Surg. 2011;10(3):171-177.
- Schottle PB, et al. Am J Sports Med. 2007;doi:10.1177/0363546506296415.
- For more information:
- Verena M. Schreiber, MD, and Shital N. Parikh, MD, FACS, can be reached at the Cincinnati Children’s Hospital Medical Center, 3333 Burnet Ave., Cincinnati, OH 45229; Schreiber’s email: verena.schreiber@cchmc.org; Parikh’s email: shital.parikh@cchmc.org.
Disclosures: Schreiber and Parikh report no relevant financial disclosures.