Allograft Osteochondral Transplantation

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CHAPTER 16 Allograft Osteochondral Transplantation

The treatment of chondral defects presents a formidable challenge. A variety of treatment methods are available, but all have shortcomings.¹ Osteochondral allografts have been used for the treatment of chondral lesions for more than 20 years.² They have the advantage of providing true articular cartilage rather than hyaline-like cartilage or fibrocartilage. They are particularly valuable when treating large defects and those with bone loss because they are not limited by size, depth, or shape of the lesion.

BASIC SCIENCE

Although the efficacy of osteochondral allografts has been shown for many years, disease transmission continues to be the area of greatest concern.³ Unfortunately, methods used to sterilize tissue have significant detrimental effects on osteochondral allografts.⁴ Sterilization methods not only devitalize all the chondrocytes, but also have negative effects on the material properties of the graft. Fortunately, the risk of disease transmission is slight if the tissue is retrieved, handled, and processed in strict accordance with standardized guidelines of the American Association of Tissue Banks (AATB).⁵

Grafts are harvested within 24 hours of the donor’s death to minimize contamination and maintain chondrocyte viability. The grafts are then processed in a clean room environment and thoroughly lavaged to remove blood components, which are the main source of pathogens and immunologic sensitization. After cultures are obtained, the grafts are treated with several antimicrobials and subsequently stored at 4° C until used.

The initial clinical series reported using grafts within 1 week from procurement because it was found that the sooner the graft is implanted, the greater the chance of chondrocyte survival.⁶^,⁷ Although the minimum chondrocyte viability for graft success is unknown, it is clear that this play a vital role in graft integrity.⁸ However, disease testing and safety precautions have resulted in tissue banks generally not releasing grafts for use until about 3 weeks. Fortunately, current storage methods are able to maintain 80% cell viability at 4 weeks.⁹^,¹⁰ In addition, the biomechanical properties of the graft are not statistically affected at that time.¹¹^,¹² However, more recent testing has shown that in commercially available grafts, a large percentage of the viable cells do not exhibit full function.¹³

Because of the limitations of fresh grafts with regard to storage and assurance of sterility, other methods of preservation, including freeze drying and fresh-frozen methods, have been evaluated.¹⁴ Unfortunately, fresh-frozen grafts have no viable chondrocytes. Freeze drying not only destroys all cells but also alters the graft’s material properties.¹⁵

Host immune response is another area of concern with allograft transplantation. It is well known that musculoskeletal allografts are capable of inducing cell-mediated and humeral immune responses in the host.¹⁶^,¹⁷ The predominant mechanism is cell-mediated, and by reducing the number of allogenic cells, the immune response would therefore be reduced. The primary source of allograft cells is the blood and bone marrow elements. The immune load is significantly reduced by removing them during graft processing,.

Chondrocytes can also evoke an immune response and matching the surface antigens has been presented as one method to reduce the load further. However, the limited number of osteochondral allografts available would make it extremely difficult to match the donor to the recipient. Fortunately, any host sensitization has not precluded favorable results. It has been suggested that because chondrocytes are embedded in the dense matrix, which acts as a barrier, intact grafts are considered immunologically privileged. However, there is a consensus that patients with autoimmune disease or inflammatory arthropathies are not appropriate candidates for osteochondral allografts. Although it may be implemented, the morbidity associated with immunosuppression does not justify its use.

PATIENT EVALUATION

History and Physical Examination

Candidates for osteochondral allografts for focal defects typically fall into three categories—osteochondritis dissecans (OCD), post-traumatic lesions, and revision surgery.¹⁸^–²⁰

Making the diagnosis of OCD by history can be difficult. Most patients with OCD have nonspecific joint pain that is insidious in onset. It is usually aggravated with activity and resolves with rest. In the pediatric population, it can often be overlooked and considered to be growing pains. Intermittent swelling may also occur if early symptoms are ignored. As the disease progresses, it may be associated with mechanical symptoms, such as catching and locking and subsequent feeling a loose body. Post-traumatic lesions and revision surgery have an obvious cause to raise awareness of articular cartilage injury. However, the symptoms are typically the same as those of OCD.

Physical examination findings may very widely. In early stages of OCD, the examination may be unremarkable. With any lesion, swelling may be present, ranging from a subtle swelling to a frank effusion. One must remember that any knee swelling in the younger patient without obvious cause should raise the suspicion of OCD, especially in a very active individual.

Pain on deep palpation of the involved area may be present. If the lesion has become partially detached, pain with clicking or popping can be found at a distinct range of pain. If the lesion has become completely separated, mechanical symptoms of a loose body may be seen. Quadriceps atrophy may be present; knowing the degree of atrophy may be helpful if the duration of symptoms is unclear.

Diagnostic Imaging

Image techniques are important not only for diagnosis, but also for treatment planning. Most osteochondral lesions can be seen on plain radiographs and should be the starting point. The initial series should include standing anteroposterior with the leg in full extension, 45-degree weight-bearing posteroanterior (PA; tunnel), lateral, and patellar sunrise views. Typically, a magnification marker is placed on the tunnel and lateral views and used as a reference if graft size matching is needed. Most OCD lesions occur on the lateral aspect of the medial femoral condyle and are most clearly seen on the tunnel view. Lateral femoral OCD lesions tend to be more posterior and are thus not always as obvious.

Plain radiographs are also used for comparison and serial following of the lesions during healing and deterioration. Standing full length films should be obtained once the diagnosis of a chondral lesion is confirmed to evaluate the mechanical axis.

Magnetic resonance imaging (MRI) is the gold standard for evaluating osteochondral and chondral lesions. If the diagnosis is in question, MRI is very sensitive for excluding or confirming pathology. If a defect is seen on plain films, MRI is recommended to determine the extent of the lesion and integrity of the articular cartilage. For example, if fluid is present behind the lesion, this indicates an unstable fragment and the need for surgery.

Bone scans (technetium bone scintigraphy) have been used to determine the extent of activity within the lesion and monitor progress of healing. However, their use has declined because of the advantages of MRI.

TREATMENT

Indications and Contraindications

Because of the risk of infection, limited availability, and cost, as well as consideration of other available treatment options, osteochondral allografts are typically reserved for chondral lesions that are 2 cm² or larger. The cause of the defect, patient age and activity level, concurrent pathology, and rehabilitation requirements are other variables that need to be considered when evaluating a potential candidate.

With regard to cause, isolated OCD and traumatic lesions have the best outcomes. Defects in the presence of diffuse degenerative changes or inflammatory arthropathies are contraindicated. Treatment of lesions secondary to avascular necrosis (AVN) may be appropriate, but only if the involvement is not progressive and the cause is understood. In addition, lesions limited to one joint surface (unipolar) fare much better than those on opposing surfaces (bipolar or kissing lesions).

When evaluating the knee, one must look beyond just the lesion. Ligament instability, absence of the meniscus, and limb malalignment have negative effects on outcome. Ligament reconstruction and meniscal allograft transplantation need to be performed concurrently or in a staged capacity. With respect to the mechanical axis, there is general agreement that if it passes through the involved compartment, realignment should be performed. However, there it is also general agreed that the degree of correction does not need to be as much as when performing an osteotomy for an arthritic knee. Typically, the aim is to pass the mechanical axis through the opposite tibial spine.

Patient age is of some debate, with most reporting an upper age limit of 40 to 45 years; it is thought that these lesions are associated with degenerative changes. However, others have implanted grafts in patients 60 years of age or older. The cause of the lesion is vital when dealing with the age factor. In the uncommon case of a mature patient with an isolated lesion and no other pathology, age alone should not be considered a contraindication. Older patients with low activity demands may be better suited for joint arthroplasty. Other relative contraindications include obesity (body mass index [BMI] >30 kg/m²), smoking, and chronic steroid use. Although sometimes taken for granted, patient expectations also need to be strongly considered. For example, it is not uncommon for patients with large lesions to regard the procedure as one that will enable them to return to unrestricted activities. Patients need to understand that high-impact activities such as distance running and contact sports should be avoided.

Conservative Management

Focal chondral defects are a common finding and can have a significant effect on limiting a patient’s activities and quality of life.²¹^,²² Symptomatic lesions can be treated nonoperatively with modification of activities, anti-inflammatories, viscosupplementation, and rehabilitation. Although these can be of benefit, many patients continue to be symptomatic.

Surgical Treatment

Several surgical options are available in addition to osteochondral allografts, but each has its advantages and disadvantages. The simplest is débridement-chondroplasty, but the benefits are commonly short-lived. Microfracture is technically easy to perform, with limited morbidity, but results in fibrocartilage filling the defect.²³^,²⁴ As a result, the outcomes deteriorate after a few years, and lesions larger than a few centimeters do poorly. Autogenous osteochondral transfer has the benefit of improving normal articular cartilage and being able to fill bone deficiencies.²⁵^,²⁶ However, donor availability is limited and it is recommended for lesions smaller than 2 to 2.5 cm². Autologous chondrocyte implantation (ACI) is often discussed as an alternative to osteochondral allografts. It can also treat large lesions and is recommended for lesions up to 16 cm².²⁷ However, it results in hyaline-like cartilage and should not be used by itself for defects more than 6 to 8 mm deep. In addition, it is technically challenging, with many published studies having a reoperation rate of 30%.

Surgical Technique

After the patient is found to meet the criteria for an osteochondral allograft, the next step is to obtain a graft (Fig. 16-1) The tissue bank used should be AATB certified to minimize the risk of infection and obtain a graft of high quality. The upper age limit of the donor of a graft is unclear. However, many surgeons recommend that the donor be younger than 35 to 40 years because of concern about tissue quality, with degenerative changes being present in older donors. Size and contour of the graft to match the recipient is another factor to be considered when selecting a graft.

FIGURE 16-1 Osteochondral lesion of the lateral femoral condyle.

Several methods can be used in an effort to match the graft and host. MRI and computed tomography (CT) scans can be used for the greatest accuracy, but plain radiographs are typically sufficient. Most commonly used are 45-degree PA and lateral views, with a magnification marker to evaluate the need for any size adjustment.

When the graft is obtained, it should be opened and inspected at the beginning of the procedure. However, because there can be a significant time delay from when the graft is requested to when it is obtained, diagnostic arthroscopy should first be performed if there is any question of change in knee pathology. Once the graft has been opened, it should be maintained in the storage solution until it is prepared. As noted, this decreases the chance of chondrocyte death, which can occur if the graft is in saline solution or exposed to the air.

The graft can be prepared using a dowel or shell configuration.²⁸^,²⁹ Dowel grafts are cylindrical plugs, often compared with the smaller autologous osteochondral autografts, and are prepared with commercially available instrumentation (Fig. 16-2). Shell grafts are manually prepared with osteotomes, saws, and rongeurs to match the defect (Fig. 16-3). Because dowel grafts are less time consuming to prepare, are easier to match, and are often press-fit, they are usually the method of choice for femoral and patellar defects. Shell grafts are generally reserved for massive grafts and those not amenable to dowel grafts. These would include tibial plateau grafts and posterior femoral condyle lesions, which cannot be reached at a 90-degree angle to the surface needed to prepare dowel grafts. In addition, if the meniscus is not intact, the tibial plateau allograft should include the meniscus. If the host meniscus is normal, the meniscus and its horn attachment are left intact.

FIGURE 16-2 Dowel graft.

FIGURE 16-3 A, Shell graft after placement in recipient site. B, Lateral view of shell graft.

The principles are the same for dowel and shell grafts. Femoral lesions can often be exposed through a miniarthrotomy, but more extensive lesions may require the exposure used for joint arthroplasty. The lesion is débrided to a vascular base to enable the graft to heal. Grafts were usually 1-cm thick or more but, with the development of dowel grafts and better fixation methods, the thickness is typically 6 to 8 mm. Basically, there should be enough bone to have the graft incorporate to the host, but not so little that the graft is unstable. If the base is not vascular at a short depth, it is extended until bleeding occurs. However, tibial plateau grafts should be at least 1 cm to have sufficient bone to enable graft fixation with screws or other means.

The initial step with the dowel method is to measure the defect using cannulated sizing cylinders (Fig. 16-4). Several systems are available, with most having cylinders in 5-mm increments ranging from 15 to 35 mm. The smallest cylinder that encompasses the defect should be selected. It is then stabilized perpendicular to the surface and a guide pin drilled through the cannulated cylinder and advanced until it is secure in the bone. Because the guide pin is used as a reference for subsequent drilling, it is crucial to have the pin at 90 degrees to the surface. Any tilt will result in the recipient site being oblique and increase the difficulty in matching the graft.

FIGURE 16-4 A, Cylindrical template used to determine size of defect. B, Smallest size found to encompass lesion is 2.5 cm.

Before reaming, a scoring instrument of the same diameter is placed over the pin and manually twisted to cut through the outer border of the articular cartilage. Its purpose is to decrease the risk of the reamer damaging the normal articular cartilage. The reamer is advanced to a depth of 6 mm (Fig. 16-5) but, if needed, reaming can continue until a bleeding base is present. Irrigation should be performed during the use of any powered instrumentation to minimize the risk of thermal necrosis. A cannulated tamp is used to compress the bottom of the defect to ensure a firm base and limit the risk of collapse. The guide pin is removed and the four quadrants of the recipient site are measured to help determine the donor graft thickness.

FIGURE 16-5 Cannulated reamer used to prepare recipient site to appropriate size and depth.

Although the graft can be prepared at the same time as the host site, it is recommended that it not be completed until the recipient site has been prepared. Although it may save time, it is much easier to make adjustments to the graft than to the host.

The allograft preparation starts by evaluation of the optimal harvest site (Fig. 16-6). The sizing cylinder is used to mark the area in the same manner as the defect. In addition, a marking pen is used to identify the 12 o’clock position of the graft to ensure correct orientation so it can be identified.³⁰ The graft is then set in the cutting jig (Fig. 16-7). The entire graft may be set into the device, but it is prudent to take the time to cut the base of the graft so that it can be secured in a position that ensures that the coring device is 90 degrees to the articular cartilage. As noted, oblique cuts can result in a graft that may be difficult to place and could result in a plug with variable thickness of articular cartilage.

FIGURE 16-6 Recipient site completed and allograft evaluated for matching donor site.

FIGURE 16-7 A, Allograft placed in cutting jig. The dowel articular surface is parallel to the base. B, Graft in cutting jig. the coring device is at 90 degrees to the articular surface.

With the graft correctly positioned, the cylindrical core reamer, with its inserted extruding rod, is used to harvest the plug. The plug is removed and marked at its four quadrants to match the recipient site (Fig. 16-8). The articular side of the graft is held in the grasping clamp, with the four marks flush with the surface of the clamp (Fig. 16-9). An oscillating saw is then used to cut the graft to the measured length.

FIGURE 16-8 Harvested plug, marked to match recipient site.

FIGURE 16-9 Grasping clamp with marks flush to surface of clamp.

The bone tamp should again be used at the recipient site prior to inserting the graft. Occasionally, the borders of the site expand just enough to impede graft placement. If the initial positioning is difficult, an arthroscopic rasp can be used to round the bottom millimeter of the bone. Caution is needed so as not to remove excess bone, which can affect the stability of the graft. Optimally, the graft should be inserted with firm finger pressure. A tamp can be gently used to assist the insertion, but forceful pressure could damage the graft.³¹ If the graft is proud by as little as 1 mm, shear forces can result in articular damage and affect the stability of the graft.³²^,³³ By contrast, if the graft is countersunk more than 1 mm, it will serve only as a bone filler. If there is insufficient bone, wafers of bone can be cut from the graft excess and used to make up the difference. If the graft is difficult to remove, one should not forcefully try to pry the graft out. A threaded K wire can be drilled in the graft for ease of removal.

If the graft is press-fit accurately, it will not require additional fixation (Fig. 16-10). If there is any question of stability, such as a graft that is not more than 75% to 80% contained, one should not hesitate to supplement fixation. Various methods have been used, but recently developed very small, compressive bioabsorbable screws are favored.

FIGURE 16-10 A, Final appearance of dowel graft secured with press fit. B, MRI scan of completed graft.

If a lesion is not circular or larger than 35 mm, multiple plugs can be used (Fig. 16-11). They are a placed in the same manner as multiple autogenous osteochondral grafts. The grafts abut each other, and attention to detail is needed to reproduce the nature curve of the surface.

FIGURE 16-11 Double plug.

If additional procedures are to be performed (e.g., meniscal allograft implantation and/or osteotomy), these are usually done at the same time as the osteochondral allograft. If the surgeon prefers to stage the procedures, the osteotomy should be done first.

Historically, there has been some debate as to when an osteotomy should be performed. However, with the less demanding techniques, there has been a shift to lower the threshold for performing an osteotomy. Conversely, the degree of correction is less than the goal when completing an osteotomy for osteoarthritis. Transferring the mechanical axis to the opposite tibial spine is commonly used as the goal. If an osteotomy is done, it should be performed on the bone opposite the allograft to avoid vascular insult.

PEARLS& PITFALLS

PEARLS

• Inspect the graft for size, shape, and quality before induction of anesthesia.

• When preparing the recipient site, always err by using a smaller size to avoid damage to normal articular cartilage. If needed, the site can always be enlarged.

• Make certain that all cuts involving the articular cartilage are perpendicular to the surface to optimize graft match and uniform thickness of the cartilage.

• When obtaining the graft, cut it several millimeters longer than the final donor length. The excess can be used to fill the recipient site if needed; it already matches the width of the plug.

• If the lesion is not circular or square, it is often better to use two smaller grafts that one large graft, which would require removal of normal articular cartilage.

PITFALLS

• The recipient should only be reamed to a depth of a 5 to 6 mm before checking the base for vascular supply. Initially, reaming to 8 mm or more may result in the unnecessary removal of host bone.

• If the graft is difficult to insert, make certain that all edges of the recipient site are cleared of debris and the end of graft is smooth, because forceful insertion will result in graft damage and cell death.

• A graft that is more than 1 mm oblique at the surface should be removed and the base adjusted to fit flush. If one attempts just to lever the graft to change its position, the tissue frequently becomes damaged; this often results in loss of press-fit fixation.

Postoperative Rehabilitation

The postoperative treatment is dependent on several variables, including the size, location, and stability of the graft, as well as additional procedures performed. It is common practice to use continuous passive motion for 6 to 8 hours daily for the initial 4 to 6 weeks, with only toe-touch weight bearing during this time. For large grafts, such as tibial plateaus, weight bearing may be limited for as long as 12 weeks. Ultimately, progression of weight bearing is decided by radiographic evidence that the graft has maintained proper position and the bone is incorporating. Activities such as swimming and low-resistance stationary bicycling are often permitted at 4 to 6 weeks. Low-impact activities with strengthening, including proprioceptive training, are gradually progressed after full weight bearing. Recreational sports are typically permitted at 4 to 6 months postoperatively. There is disagreement as to whether patients treated for OCD or well-contained isolated lesions should be active in high-impact activities or sports. However, there is general agreement that in salvage cases, these types of activities should be avoided.

OUTCOMES

Osteochondral allograft success for treating chondral lesions has been well documented in the orthopedic literature. In addition, the follow-up of these studies is of longer duration than that for the other biologic procedures. There have been several published reports, but the University of Toronto and University of California at San Diego have had the most experience.

The University of Toronto began its osteochondral allograft program in 1972 and reported the results of their first 100 patients in 1985.³⁴ These patients encompassed a wide variety of pathology and had variable outcomes. However, it was found that using shell grafts for post-traumatic lesions resulted in a 75% success rate (36 of 48). They subsequently published their intermediate and long-term results using grafts for post-traumatic knee lesions. Of the 126 knees in 122 patients treated, 108 (86%) were successful.³⁵ The average follow-up was 7.5 years (range, 2 to 22 years), with a survivorship of 95% at 5 years, 85% at 10 years, and 74% at 15 years for femoral condyle grafts. For tibial plateau grafts, the survivorship was 95% at 5 years, 80% at 10 years, and 65% at 15 years.³⁶ In 2008, the University of Toronto reported their findings from a critical analysis of the 69 known failures treated at their institution.³⁷ They found that the grafts that failed early (less than 1 year) lacked viable chondrocytes and cartilage matrix staining. The importance of proper graft handling and timely implantation to maintain chondrocyte viability and graft quality was stressed. Analyses also showed a trend toward better survivorship with adjunct meniscal allograft transplantation and realignment osteotomies. Finally, the importance of mechanical stability and fixation was evident. It was found that graft instability leads to nonunion and continued remodeling at the host-graft interface.

The University of California at San Diego (UCSD) instituted their use of osteochondral allografts in 1983. In 1999, they published the results of 211 patients treated for defects secondary to OCD, trauma, or AVN.³⁸ The mean follow-up was 52 months (range, 12 to 186 months). Good to excellent results were reported in 177 of 211 patients (84%). Analyses of the treated knee lesions showed success in 116 of 125 (93%) femoral, 26 of 40 (65%) tibiofemoral, and 35 of 46 (76%) patellofemoral grafts. Uncorrected ligamentous instability, limb malalignment, and bipolar lesions were found to be associated with increased failure rates. In 2007, UCSD published their experience of treating OCD lesions with osteochondral allografts.³⁹ In 64 patients, 66 knees were treated, with a mean allograft size of 7.5 cm². The mean follow-up was 7.7 years (range, 2 to 22 years). Only one patient was lost to follow-up. Of the remaining 65 knees, 47 (72%) were rated good to excellent, 7 (11%) were rated fair, and 1 (2%) was rated poor. An additional 10 patients(15%) underwent reoperation.