Introduction

The clinical consequences of articular cartilage defects of the knee are pain, swelling, mechanical symptoms, athletic and functional disability, and osteoarthritis. Full-thickness articular cartilage defects have a poor capacity to heal because of the cartilage’s isolation from systemic regulation and its lack of vessels and nerve supply. The challenge to restore the articular cartilage surface is a multidimensional task faced by both basic scientists in the laboratory and orthopedic surgeons in the operating room. A growing number of patients are presenting for consultation in order to maintain their active lifestyles and hobbies. These patients wish to continue activities that have, in the past, been achievable only for younger and healthier knees. Since the late 1970s, different techniques to address articular cartilage injuries and defects have emerged as valid therapeutic options. Although options to treat these lesions have expanded, the difficulty facing practitioners is choosing which technique to best address the defects of each individual patient.

Although the regeneration of true hyaline cartilage is not yet a reality, a variety of methods have the potential to stimulate the formation of a new articular surface, including microfracture of subchondral bone, use of auto- or allografts, cell transplantation, targeted growth factors, and artificial matrices. Reports of the clinical results of these procedures have documented clinical improvement for most of the patients.^1,2,3,4 However, despite the availability of all of these techniques and the advances in imaging that have led to an increased understanding of the frequency and types of chondral lesions, patient evaluation and treatment selection still remain challenging. In evaluating a patient for cartilage repair, one must characterize not only the cartilage lesion itself but the various clinical factors and comorbidities embodied by each individual.

Several comorbidities such as ligamentous instability, deficient menisci, or malalignment of the mechanical limb axis or extensor mechanism often coexist with the articular surface pathology. Moreover age-related, nonprogressive, superficial fibrillation of cartilage and focal lesions of the articular surface must be distinguished from degeneration of cartilage occurring as a part of syndrome of osteoarthritis.⁵ As a consequence, the clinician must define, characterize, and classify local, regional, and systemic, medical, and family history factors that may influence the progression, degeneration, or regeneration of the defect. Careful patient evaluation is essential in selecting the proper treatment plan: lesions with different etiology and size require different treatments and the comorbidities may need to be treated in conjunction with symptomatic chondral injuries to provide a mutually beneficial effect. Thus, the evaluation and characterization of the patient as a whole is key to optimizing the results of surgery.

This chapter provides the guidelines for selecting the proper treatment algorithm.

Clinical Management

The initial step in the workup is the history. This should include mechanism of injury, time course, and quality of symptoms; review of previous treatment; and the effects of those treatments. Peterson et al. found that the average patient presenting for cartilage restoration had 2.1 previous treatments, usually with a different physician.⁶ In this setting, access to operative reports, pervious imaging, and even direct communication with previously treating surgeons can provide information.

During the physical examination, the surgeon should be careful not to assume that the articular cartilage lesion is responsible for all symptoms but should attempt to delineate concomitant pathologies that may be contributing symptoms. It is important to recognize that not all chondral lesions cause symptoms. Conversely, not all symptoms are related to the chondral or osteochondral defect. Often concomitant pathology exists and can play a role in the symptoms that the patient maybe experiencing. In addition to the sites of point tenderness, effusion, crepitus, and catching, the examination should carefully assess alignment, range of motion, and patellofemoral tracking. Evaluation for ligamentous integrity is also valuable in considering concomitant pathologies of the knee. Other mechanical issues of obesity and gait patterns may exclude a patient from certain treatments because of potential inability to comply with often extensive rehabilitation protocols.

Required radiographs include standing anteroposterior, lateral, patellar skyline, a 45-degree flexion posterior anterior weight-bearing view, and a full-length alignment film. No cartilage restoration procedure should be performed in the setting of malalignment; therefore, if the mechanical axis bisects the affected compartment, a corrective osteotomy should be strongly considered as a concomitant or staged procedure.^7,8

Access to MRI is important in developing and executing an effective clinical approach to cartilage repair surgery.

With a high-resolution fast spin echo sequencing technique in the saggital, coronal, and axial planes, articular cartilage surfaces can be well imaged and measured. This allows accurate characterization of not only the lesions in question but the state of all opposing cartilage surfaces and menisci.

Quantitative MRI techniques, such as T2 mapping, T1rho, and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), provide noninvasive information about cartilage and repair tissue biochemistry.

Diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) demonstrate information regarding the regional anisotropic variation of cartilage ultrastructure.⁹ These advantages provide preoperative information and may allow for a postoperative assessment of actual glycosaminoglycan content of repaired or replaced tissue.¹⁰

If the lesion involves the subchondral bone, then computer tomography scanning may also be necessary to assess defect geometry and depth, especially in the presence of osteochondral defects that may require bone grafting in addition to an articular cartilage restorative procedure.

An examination under anesthesia is required to better assess the knee instability. This is performed routinely before every knee arthroscopy. The first operation after the diagnosis of an articular cartilage defect is often not the definitive procedure. At times, arthroscopy is performed initially as a diagnostic tool to assess the lesion, in terms of its location, geography, surface area, and depth. The surrounding articular surfaces in the uninvolved compartments, the state of the menisci, and the presence or absence of additional pathology need also to be defined. If one is considering definitive treatment with autologous chondrocyte implantation (ACI), a biopsy should be performed at this time. Similarly, if a significant subchondral defect exists, primary bone grafting can be performed at the index operation.

Local and Regional Facts

To ensure uniform standards of evaluating articular cartilage repair, a universally accepted classification system is necessary. Many different grading systems for cartilage defects are cited in the literature, including those of Outerbridge,¹¹ Insall,¹² Bauer and Jackson,¹³ and Noyes.¹⁴ To avoid confusion, the International Cartilage Repair Society (ICRS) has developed a grading system to be used as a universal language when surgeons are communicating about cartilage lesions. The ICRS characterizes Grade 0 as normal, Grade 1 as superficial lesions and fissures, Grade 2 as lesions extending down to less than 50% of the cartilage depth, and Grade 3 as all lesions extending more than 50% of the cartilage depth and down to bone. Finally, Grade 4 lesions are all that extend down through the subchondral bone plate (Fig. 2-1). This grading system is also included in a comprehensive method of documentation and classification, which encompasses a global description of not only the lesion but all of the local factors and comorbidities previously discussed.¹⁵ The following variables are included in the standards:

FIGURE 2-1 ICRS cartilage grading score.

A comprehensive analysis of the local and regional factors related to an articular cartilage lesion is utilized to develop a treatment plan. A flowchart has been created to summarize primary treatment options and secondary treatment options. There are separate charts for femoral and patellar defects. Primary treatment options should be considered first-line treatment choices. Secondary treatment options are considered if primary treatment fails or if other factors prevent the use of a primary treatment option (Figs. 2-2 and 2-3).

FIGURE 2-2 Algorithm for femoral defects. ACI, Autologous chondrocyte implantation; OCG, osteochondral grafts.

FIGURE 2-3 Algorithm for patellar and trochlear defects. ACI, Autologous chondrocyte implantation; ACI 2-3, autologous chondrocyte implantation generation 2 and 3; PF, patella femoral realignment.

Emphasis must again be placed on the importance of looking at the entire picture when characterizing a cartilage lesion. It is imperative to consider each lesion in the context of alignment, ligamentous, and meniscal integrity (macroenvironment), as well as molecular-level factors such as chondrocyte function, synovium, chondropenia, and cartilage integrity (microenvironment).

The Clinical Algorithm: the Chondropenic Pathway

After completion of the comprehensive assessment described earlier, patients can then be stratified a clinical algorithm. This chondropenic pathway has been developed for the management of articular cartilage defects. The algorithm defines 10 patient-directed situations based on lesion size, depth, and associated issues such as alignment, ligament, and meniscal integrity. Each situation considers the problem category, the therapeutic options, and the current unresolved issues.

Conclusions and Future Challenges

The challenges of articular cartilage repair and restoration continue despite recent advances. Marrow stimulation techniques, substitution replacement options, and biological replacement options each have a role in the treatment algorithm of articular cartilage defects. This treatment algorithm must take into account not only the characterization of the specific cartilage lesion but also the myriad of local and regional factors as well as the comorbidities attached to each patient. As of yet no single treatment option can reestablish the hyaline cartilage seen in normal articular cartilage. The goal for future treatments is to develop new technologies and disease-modifying interventions that protect and preserve the joint over time by maintaining biochemical, biomechanical, and cellular integrity. Until these technologies exist, collaboration between the basic scientist and the clinician will continue to advance our current technologies in an effort to restore the violated articular cartilage surface.