Knee Osteoarthritis: Symptoms, Staging, and Treatment Options

Anatomy of the Knee: Understanding What Breaks Down

The knee is the largest and most complex joint in the human body. Understanding its basic anatomy helps explain why OA develops where it does and why certain treatments target specific structures.

The knee has three compartments:

• Medial compartment: The inner side of the knee, where the medial femoral condyle meets the medial tibial plateau. This is the most common site for knee OA, partly because it bears roughly 60-70% of the load during normal walking [1].
• Lateral compartment: The outer side of the knee. OA here is less common but can occur, especially in people with valgus (knock-knee) alignment.
• Patellofemoral compartment: Where the kneecap (patella) glides over the front of the femur. Patellofemoral OA often causes pain going up and down stairs, squatting, and sitting for prolonged periods.

Several key structures are involved in knee OA:

• Articular cartilage: The smooth, white tissue (2-4 mm thick) that covers the ends of the femur, tibia, and back of the patella. This is the primary tissue that degrades in OA.
• Menisci: Two C-shaped fibrocartilage pads (medial and lateral) that sit between the femur and tibia, acting as shock absorbers and load distributors. Meniscal tears are both a cause and consequence of knee OA [2].
• Synovial membrane: The tissue lining the inside of the joint capsule that produces synovial fluid. In OA, this membrane becomes inflamed (synovitis), producing excess fluid and inflammatory molecules.
• Subchondral bone: The bone just beneath the cartilage. In OA, this bone undergoes remodeling, becoming sclerotic (hardened) and sometimes developing cysts and bone marrow lesions, which are associated with pain [3].

How OA Develops in the Knee

Knee OA typically begins with a triggering event or process, often a combination of genetic predisposition, mechanical factors, and inflammatory biology. The disease progresses through a recognizable sequence, though the rate of progression varies enormously between individuals.

The early phase involves softening of the cartilage surface (chondromalacia), often without symptoms. Proteoglycans, the molecules that give cartilage its ability to resist compression, begin to degrade. The collagen network, which provides tensile strength, develops surface fibrillation (fraying).

As the cartilage surface breaks down, fragments enter the synovial fluid and trigger an inflammatory response in the synovial membrane. This synovitis produces additional inflammatory cytokines that accelerate cartilage destruction, creating a self-perpetuating cycle [4]. The joint space begins to narrow as cartilage thins.

In later stages, cartilage is lost completely in some areas, exposing subchondral bone. Bone responds by becoming denser (sclerosis) and forming osteophytes (bone spurs) at the joint margins. The joint may develop angular deformity, typically a varus (bow-legged) alignment as the medial compartment narrows preferentially.

The Kellgren-Lawrence Grading System

The Kellgren-Lawrence (KL) grading system is the most widely used classification for knee OA severity. It is based on plain X-ray findings and was first described in 1957 [5]. While imperfect, it remains the standard language for describing OA severity in clinical practice and research.

Grade 0: Normal

No radiographic features of OA. Joint space is normal. No osteophytes. This does not mean the joint is necessarily healthy: early cartilage changes can be present on MRI that are invisible on X-ray.

Grade 1: Doubtful

Possible narrowing of the joint space, with tiny osteophytes of doubtful significance. Many radiologists and clinicians debate whether this represents early OA or a normal variant. Symptoms at this stage, if present, are typically mild and intermittent: occasional aching after heavy activity that resolves with rest.

Grade 2: Mild

Definite osteophytes visible on X-ray with possible joint space narrowing. This is the earliest stage that most clinicians would confidently diagnose as OA. Symptoms usually include stiffness after prolonged sitting, aching during or after exercise, and occasional swelling after heavy use. Many people at this stage can still do most activities with minor modification.

Grade 3: Moderate

Multiple moderate-sized osteophytes, definite joint space narrowing, some sclerosis of subchondral bone, and possible deformity of bone ends. Symptoms typically include daily pain, significant morning stiffness (usually less than 30 minutes, which distinguishes OA from rheumatoid arthritis), difficulty with stairs and squatting, and crepitus (grinding sensation or sound) during movement. Walking distance often becomes limited.

Grade 4: Severe

Large osteophytes, marked joint space narrowing or obliteration (bone-on-bone), severe sclerosis, and definite bony deformity. Pain is usually constant, present at rest and at night. Walking is significantly limited. The joint may feel unstable, and angular deformity is often visible.

Imaging: X-Ray vs. MRI

Standard weight-bearing X-rays remain the first-line imaging for knee OA. They are inexpensive, widely available, and show joint space narrowing, osteophytes, and bony changes clearly. Weight-bearing views (taken while standing) are essential; non-weight-bearing X-rays can miss significant narrowing because the bones are not compressed together [6].

MRI provides substantially more information. It directly visualizes cartilage (X-rays only infer cartilage thickness from joint space width), menisci, ligaments, synovial inflammation, and bone marrow lesions. MRI can detect OA changes years before they appear on X-ray [7]. It is particularly valuable for:

• Detecting early cartilage damage in patients with symptoms but normal X-rays
• Evaluating meniscal tears that may be contributing to symptoms
• Identifying bone marrow lesions, which correlate with pain and predict structural progression
• Assessing which compartment is most affected, which is critical for surgical planning
• Monitoring response to regenerative treatments

The limitation of MRI is cost and the potential for finding incidental abnormalities that cause unnecessary concern. Not every MRI finding is clinically significant, and treatment should be guided by the combination of imaging findings and clinical symptoms.

Treatment Ladder: Early to Advanced

For Early Knee OA (KL Grade 1-2)

The window for disease modification is widest at this stage. Interventions here have the greatest potential to change the long-term trajectory of the disease.

Exercise and Physical Therapy: Quadriceps strengthening is the single most impactful intervention. A structured program targeting the quadriceps, hamstrings, hip abductors, and calf muscles reduces pain, improves function, and may slow structural progression. Aerobic fitness training (walking, cycling, swimming) provides additional benefits [8].

Weight Management: For patients who are overweight, losing 10% of body weight reduces knee pain by approximately 50%. The mechanical and inflammatory benefits of weight loss are substantial, and every pound lost reduces the force across the knee by 3-6 pounds during walking [9].

Activity Modification: This does not mean stopping activity. It means substituting high-impact activities with lower-impact alternatives during symptomatic periods. A runner might switch to cycling; a basketball player might transition to swimming.

Topical NSAIDs: Diclofenac gel applied directly to the knee provides anti-inflammatory effects with minimal systemic absorption. It is recommended as a first-line pharmacological option by the ACR [10].

For Moderate Knee OA (KL Grade 2-3)

At this stage, the goal shifts toward aggressive symptom management while continuing disease-modification efforts.

Platelet-Rich Plasma (PRP): The evidence for PRP in knee OA is strongest at this stage. A meta-analysis of 30 RCTs found PRP superior to hyaluronic acid and corticosteroid injections for both pain and function at 6 and 12 months [11]. Leukocyte-poor PRP appears to produce better outcomes than leukocyte-rich formulations for intra-articular use. A typical protocol involves 1-3 injections spaced 2-4 weeks apart.

Hyaluronic Acid Injections: Viscosupplementation replaces the degraded hyaluronic acid in OA synovial fluid, improving joint lubrication and potentially reducing inflammation. Results are modest on average, but some patients report significant relief lasting 6-12 months. Higher molecular weight products may be more effective [12].

Unloader Braces: For predominantly medial or lateral compartment disease, an unloader brace shifts weight away from the affected compartment. They can reduce pain and improve function, though compliance is sometimes an issue due to bulk and discomfort.

Corticosteroid Injections: Appropriate for acute flares, providing rapid anti-inflammatory relief. Limit to 3-4 per year maximum, given evidence that repeated injections may accelerate cartilage loss [13].

For Advanced Knee OA (KL Grade 3-4)

Cartilage is significantly or completely lost in one or more compartments. Regenerative therapies can still improve symptoms but are less likely to produce structural cartilage repair at this stage.

Mesenchymal Stem Cell (MSC) Injections: Some patients with advanced OA report significant pain improvement after MSC injections, even when cartilage regeneration is not expected. The anti-inflammatory and immunomodulatory effects of MSCs may provide benefit independent of structural repair [14].

Genicular Nerve Block or Ablation: Genicular nerve radiofrequency ablation targets the sensory nerves that transmit pain from the knee joint. It can provide significant pain relief for 6-12 months and may be repeated. This does not treat the underlying disease but can be valuable for patients who are not surgical candidates or want to delay surgery [15].

Partial Knee Replacement (Unicompartmental Arthroplasty): When OA is limited to a single compartment (most commonly the medial), a partial knee replacement replaces only the damaged section. Recovery is faster than total knee replacement, the joint feels more natural, and outcomes in well-selected patients are excellent. The key is ensuring the other compartments and the cruciate ligaments are intact [16].

Total Knee Replacement (TKA): For diffuse, end-stage disease affecting multiple compartments, TKA remains the definitive treatment. Modern implants and surgical techniques have improved outcomes significantly. About 85-90% of patients report substantial pain relief and improved function. Prosthetics last 15-25 years in most cases, which is an important consideration for younger patients who may face revision surgery later in life [17].

Regenerative Options Specific to the Knee

The knee is the best-studied joint for regenerative medicine, and several treatments show particular promise in this location:

Bone Marrow Aspirate Concentrate (BMAC): A concentrated form of bone marrow containing MSCs, growth factors, and anti-inflammatory cytokines. BMAC is typically harvested from the iliac crest (hip bone) and injected into the knee in the same procedure. It combines the benefits of PRP and stem cell therapy and has growing clinical evidence for knee OA [18].

Micro-fragmented Adipose Tissue: Fat tissue is harvested via liposuction, mechanically processed to concentrate the stromal vascular fraction (which contains MSCs and growth factors), and injected into the knee. Early clinical data shows promising pain and functional improvements at 1-2 year follow-up [19].

Prolotherapy: Dextrose prolotherapy injections around the knee joint and supporting ligaments can reduce pain and improve function. A randomized trial found sustained benefits at 1-year follow-up [20].

When to Consider Knee Replacement

The decision to proceed with knee replacement should be based on several factors:

• Functional limitation: Inability to walk reasonable distances, climb stairs, or perform daily activities despite adequate conservative treatment
• Pain severity: Consistent pain at rest or at night that interferes with sleep and quality of life
• Failed conservative treatment: Genuine trial of physical therapy (at least 6-12 weeks of structured exercise), weight management, appropriate injections, and activity modification
• Radiographic severity: Imaging confirming advanced structural damage consistent with symptoms
• Overall health: Medical fitness for surgery and rehabilitation

Age alone is not a reason to delay surgery. While surgeons once told patients to “wait as long as possible” because prosthetics had limited lifespans, modern implants last significantly longer. Waiting too long can lead to deconditioning, muscle atrophy, and fixed deformities that make surgery more complex and recovery harder.

Conversely, surgery should not be rushed. Research shows that up to 20% of knee replacement patients are dissatisfied with their outcome, and preoperative factors like realistic expectations, adequate quadriceps strength, and mental health optimization improve results [21].

References

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[3] Felson DT, Chaisson CE, Hill CL, et al. The association of bone marrow lesions with pain in knee osteoarthritis. Ann Intern Med. 2001;134(7):541-549. doi:10.7326/0003-4819-134-7-200104030-00007. PMID: 11281736

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[9] Messier SP, Mihalko SL, Legault C, et al. Effects of intensive diet and exercise on knee joint loads, inflammation, and clinical outcomes among overweight and obese adults with knee osteoarthritis: the IDEA randomized clinical trial. JAMA. 2013;310(12):1263-1273. doi:10.1001/jama.2013.277669. PMID: 24065013

[10] Kolasinski SL, Neogi T, Hochberg MC, et al. 2019 American College of Rheumatology/Arthritis Foundation guideline for the management of osteoarthritis of the hand, hip, and knee. Arthritis Care Res. 2020;72(2):149-162. doi:10.1002/acr.24131. PMID: 31908163

[11] Belk JW, Kraeutler MJ, Houck DA, et al. Platelet-rich plasma versus hyaluronic acid for knee osteoarthritis: a systematic review and meta-analysis of randomized controlled trials. Am J Sports Med. 2021;49(1):249-260. doi:10.1177/0363546520909397. PMID: 32302218

[12] Altman RD, Manjoo A, Fierlinger A, et al. The mechanism of action for hyaluronic acid treatment in the osteoarthritic knee: a systematic review. BMC Musculoskelet Disord. 2015;16:321. doi:10.1186/s12891-015-0775-z. PMID: 26503103

[13] McAlindon TE, LaValley MP, Harvey WF, et al. Effect of intra-articular triamcinolone vs saline on knee cartilage volume and pain in patients with knee osteoarthritis: a randomized clinical trial. JAMA. 2017;317(19):1967-1975. doi:10.1001/jama.2017.5283. PMID: 28510679

[14] Mancuso P, Raman S, Glynn A, et al. Mesenchymal stem cell therapy for osteoarthritis: the critical role of the cell secretome. Front Bioeng Biotechnol. 2019;7:9. doi:10.3389/fbioe.2019.00009. PMID: 30761299

[15] Choi WJ, Hwang SJ, Song JG, et al. Radiofrequency treatment relieves chronic knee osteoarthritis pain: a double-blind randomized controlled trial. Pain. 2011;152(3):481-487. doi:10.1016/j.pain.2010.09.029. PMID: 21055873

[16] Liddle AD, Judge A, Pandit H, Murray DW. Adverse outcomes after total and unicompartmental knee replacement in 101,330 matched patients: a study of data from the National Joint Registry for England and Wales. Lancet. 2014;384(9952):1437-1445. doi:10.1016/S0140-6736(14)60419-0. PMID: 25012116

[17] Evans JT, Walker RW, Evans JP, et al. How long does a knee replacement last? A systematic review and meta-analysis of case series and national registry reports with more than 15 years of follow-up. Lancet. 2019;393(10172):655-663. doi:10.1016/S0140-6736(18)32531-5. PMID: 30782341

[18] Shapiro SA, Kazmerchak SE, Heckman MG, et al. A prospective, single-blind, placebo-controlled trial of bone marrow aspirate concentrate for knee osteoarthritis. Am J Sports Med. 2017;45(1):82-90. doi:10.1177/0363546516662455. PMID: 27566242

[19] Tremolada C, Colombo V, Ventura C. Adipose tissue and mesenchymal stem cells: state of the art and Lipogems technology development. Curr Stem Cell Rep. 2016;2:304-312. doi:10.1007/s40778-016-0053-5

[20] Rabago D, Patterson JJ, Mundt M, et al. Dextrose prolotherapy for knee osteoarthritis: a randomized controlled trial. Ann Fam Med. 2013;11(3):229-237. doi:10.1370/afm.1504. PMID: 23690322

[21] Bourne RB, Chesworth BM, Davis AM, et al. Patient satisfaction after total knee arthroplasty: who is satisfied and who is not? Clin Orthop Relat Res. 2010;468(1):57-63. doi:10.1007/s11999-009-1119-9. PMID: 19844772

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