Introduction

Large osteochondral defects in the weight-bearing zone of the medial or lateral femoral condyle, especially in young patients, create a challenge for orthopaedic surgeons. Addressing both the cartilaginous and the osseous defect, mosaicplasty represents a modified technique of the conventional osteochondral autologous transplantation system (OATS) technique by transplantation of multiple OATS plugs into the defect area.^1,2 However, this technique is limited because of the availability of the plugs at the donor site. Additionally, a primary stability of multiple plugs as well as restoration of the anatomical congruence at the cartilaginous surface level is technically demanding and often infeasible.

In the 1960s and 1970s, the posterior femoral condyle of the knee was already recognized as a potential donor site for osteochondral tissue.^3,4 The transfer of the posterior femoral condyle using an anterior approach was started in the 1990s at our department at klinikum Rechts der Isar in Munich, Germany. The autologous Mega-OATS procedure is a technical advancement of the posterior femoral condyle transfer.^5,6 Contrarily, the allogenic Mega-OATS technique illustrates an alternative procedure sparing the posterior femoral condyle as the donor site.⁷ In both autologous and allogenic Mega-OATS techniques, an almost anatomical restoration of the condyle curvature as well as an optimal congruence to the adjacent cartilaginous surface can be achieved. Nevertheless, comorbidities such as malalignment and ligamentous instabilities must be addressed before or ideally simultaneously with the Mega-OATS procedure.

Isolated Mega-OATS procedures as well as Mega-OATS procedures combined with additional procedures (malalignment correction, ligamentous stabilization, meniscus surgery, etc.) were clinically and imaging based (radiological, MRI) reevaluated in the short- and mid-term follow-up. Overall, the reevaluations demonstrated a considerable improvement regarding function, pain reduction, and swelling as well as resumption of sporting activities compared to preoperatively.^5,6,8,9

Indications

The indication criteria for the Mega-OATS technique are similar to the OATS procedure. However, because of the size of the osteochondral lesion, a conventional single plug OATS technique or a press-fit technique of a limited number of osteochondral cylinders is insufficient for adequate coverage of the defect. Therefore, the Mega-OATS technique is an alternative procedure to the mosaicplasty in which multiple osteochondral plugs are transplanted.

The indication criteria of the Mega-OATS procedure include the following:

Contraindications

The contraindication criteria for the Mega-OATS technique include the following:

Surgical Technique

Surgical Equipment

• Standard surgical equipment for knee surgery including condyle retractors

• Blade chisels of various widths

• Mega-OATS devices including the Mega-OATS workstation (Arthrex, Naples, Fl.) (Fig. 6-1, A-D)

• Arthroscopic devices for concomitant and arthroscopically addressable lesions, if necessary

FIGURE 6-1 A, Specially designed Mega-OATS instruments. B, Devices for measuring the osteochondral defect size. Preparation of the recipient socket and the Mega-OATS cylinder (20 to 35 mm diameter). C, Special workstation for harvest of the Mega-OATS cylinder. D, Devices for implantation of the Mega-OATS cylinder into the recipient socket or readjusting the Mega-OATS cylinder within the recipient socket.

Exposure

Before the Mega-OATS procedure, a primary arthroscopy for addressing concomitant lesions is optional (e.g., meniscal and cruciate ligament pathologies). Therefore, a standard anterolateral parapatellar portal for the camera and an anteromedial parapatellar portal for the arthroscopic devices are applied.

Alternatively, a central skin incision may be performed ahead of the arthroscopy, sparing separate skin incisions for the arthroscopic procedure.

In detail, the central skin incision is performed starting at the patella and ending at the tibial tuberosity. In cases of an additional high tibial osteotomy, the skin incision may be extended distally.

According to the localization of the defect, an anteromedial (conventional, alternatively minimally invasive subvastus approach) or anterolateral arthrotomy of the knee joint is accomplished.

Partial resection of the Hoffa’s fat pad is occasionally necessary for sufficient exposure of the osteochondral lesion.

In a maximal flexed knee position, the border of the osteochondral defect at the medial or lateral femoral condyle is marked using a sterile pen (Fig. 6-2).

FIGURE 6-2 Exposition of the large osteochondral defect within the weight-bearing zone of the medial femoral condyle following anteromedial arthrotomy.

Mega-OATS Harvest, Preparation, and Implantation

The defect size is measured with a ruler or the Mega-OATS template (Fig. 6-3, A).

FIGURE 6-3 Preparation of the recipient socket. A, Measurement of the osteochondral size of the defect. B, Hollow drill for preparation of the defect bed. C, Measurement of the defect socket.

A K-wire is perpendicularly positioned in the center of the lesion, serving as a guidance device for the hollow drill (diameter 20 to 35 mm) (Fig. 6-3, B).

The hollow drill creates a stable osteochondral skirt around the defect.

A trephine, equally sized as the hollow drill, is used for preparation of the recipient socket.

Sterile pads may be utilized to avoid dispersion of drilling debris into the joint.

The depth of the socket is defined by the extension of the osseous defect or the necrotic zone. Intraoperatively, the correct drilling depth is accomplished by occurrence of bleeding of the subchondral surfaces within the defect socket. Then, the size of the transplant cavity is measured with the Mega-OATS depth measurement guide (Fig. 6-3, C).

A cancellous bone plasty is essential if the depth of the cavity exceeds the depth of the transplant. Additionally in cases of a sclerotic defect bed, multiple drillings may be necessary for sufficient healing.

According to the osteocartilaginous defect size and depth, the posterior femoral condyle is harvested by a blade chisel osteotomy in a maximally flexed knee position.

In detail, the direction of the posterior femoral condyle osteotomy is parallel or in elongation to the posterior cortex of the femoral diaphysis (Fig. 6-4, A-B).

FIGURE 6-4 Osteotomy of the posterior femoral condyle. A and B, The osteotomy is performed parallel or in elongation of the cortex of the posterior femoral diaphysis. C, Temporary fixation of the incomplete posterior femoral condyle transplant using a K-wire during osteotomy for prevention of transplant slippage.

To keep the correct plane with the blade chisel from sliding out, the cut of the blade must be turned regularly.

To secure the posterior femoral condyle against transplant slippage, the posterior condyle may be temporarily fixed with a K-wire before completion of the osteotomy (Fig. 6-4, C).

Condyle or Hohmann retractors are used for protection of the posterior capsule, including the neurovascular structures, the intercondylar soft tissue, and the collateral ligament.

In cases of critical size of the osteochondral defect area (30 to 35 mm diameter), the harvesting of the posterior femoral condyle may occur before the preparation of the defect bed for securing of an adequately sized Mega-OATS cylinder.

The Mega-OATS cylinder is harvested out of the removed posterior femoral condyle using a specially designed Mega-OATS workstation. The osteotomized posterior femoral condyle is fixed by screws at the workstation (Fig. 6-5, A).

FIGURE 6-5 Mega-OATS cylinder harvest out of the osteotomized posterior femoral condyle. A, Fixation of the posterior femoral condyle within the Mega-OATS workstation. B, Perpendicular harvest of the Mega-OATS cylinder out of the fixed posterior femoral condyle. C, Final harvest of the Mega-OATS cylinder.

For optimal harvest of the Mega-OATS cylinder, the guiding device of the workstation is adapted perpendicularly to the cartilage surface of the transplant (Fig. 6-5, B).

Then the Mega-OATS cylinder is cut out of the transplant using a core drill (Fig. 6-5, C). Usually, a diameter of the Mega-OATS cylinder between 20 to 35 mm can be achieved depending of the size of the osteochondral defect and the height of the patient.

The prepared Mega-OATS cylinder is finally implanted in the recipient socket following determination of the socket depth. Because the diameter of the Mega-OATS cylinder is 0.4 to 0.5 mm larger than the diameter of the defect socket, a press-fit technique is usually feasible (Fig. 6-6).

FIGURE 6-6 Positioning of the Mega-OATS cylinder into the recipient socket with adjustment of an optimal surface congruency to the adjacent cartilage.

When an osteochondral skirt is less than 75% of the recipient socket circumference or when there are irregularities in the cavity, a temporary fixation of the Mega-OATS cylinder using a small fragment screw may be necessary. In these rare cases, the screws should be removed arthroscopically after 6 week before resumption of partial weight bearing.

For optimal congruency of the Mega-OATS cylinder to the adjacent cartilage curvature, the cylinder must be usually rotated about 90° to the longitudinal central axis of the cylinder to match the different surface curvatures of the posterior part of the femoral condyle and the weight-bearing zone of the femoral condyle.

An intraarticular drainage is placed before closure of the arthrotomy in a layer-by-layer fashion. Finally, sterile wound dressing and elastocompressive wrapping are applied.

Comorbidities

Deviation of the axis of the leg (malalignment), ligamentous instability, meniscal lesions, or accompanying smaller osteochondral lesions are common comorbidities of large osteochondral defects in the weight-bearing zone of the knee joint. Usually, these comorbidities are treated simultaneously in a single-stage surgical intervention.

In cases of deviation of the axis and consecutive mechanical overload of the impaired compartment, a simultaneous correction of the axis is essential.

Ligamentous instability should also be addressed at the same time to avoid shear forces at the osteochondral transplant. Separate smaller osteochondral lesions may be treated with additional OATS plugs.

Postoperative Management

Unloading of the operated lower extremity for 6 weeks postoperatively is mandatory.

Within this period, active range of motion is limited to flexion/extension 90°-0°-0° and supported by usage of a 4-point hard frame knee brace for at least 6 weeks.

Continuous passive motion therapy and isometric muscle strengthening are concomitantly recommended.

Beginning at the 7th week postoperatively, progressive loading with 20 kg per week and free active range of motion are allowed as tolerated until full weight bearing of the extremity is achieved. Within the 3rd to 6th postoperative month, progressive muscle and proprioceptive training are performed.

A specific sports rehabilitation program is started at the 7th postoperative month. Sports at a recreational competition level in low- and mid-impact sports (with respect to the contraindication criteria) may be restarted at 10 to 12 months postoperatively; however, professional competition levels in mid- and high-impact sports are not recommended.

Postoperative Imaging

Postoperative standard radiography of the knee joint is performed after removal of the drain. A routine MRI postoperatively is not necessary but may be indicated in prolonged postoperative pain and swelling as well as for evaluation of osseous ingrowth, chondral congruency, and graft viability (Fig. 6-7, A-B). For special evaluation of graft viability, MRI may be enhanced using an intravenous fluid application.

FIGURE 6-7 Postoperative MRI of the Mega-OATS transplant: A coronal and, B sagittal. The cylinder shows osseous integration, an adequate restoration of the condyle curvature, and a congruent chondral surface to the surrounding cartilage (arrows).

Conclusion

The Mega-OATS procedure represents a reproducible technique that allows a restoration of pain-free or low-pain-level daily mobility and in many cases enables one to restart sporting activities on a recreational level.

However, this technique has to be considered as a salvage procedure for young individuals with large osteochondral defects in the weight-bearing zone of the femoral condyle, who are considerably limited in performing normal daily and sporting activities. Addressing concomitant knee pathologies is essential for obtaining overall satisfactory results.