(This section is primarily for the use of Medical Practitioners.)

The KOGS is a dynamic radiological tool for degenerative osteoarthritis (OA) of the knee. Due to its progressive staging of degeneration in evaluating all three compartments, it aids the surgeon in deciding on the type and timing of knee arthroplasty. The system complements clinical evaluation as a tool to improve the surgical decision.

To improve the identification of the correct knee for a specific arthroplasty, the following need to be evaluated:
· Tibiofemoral joints (TF) and
· Patellofemoral joints (PFJ).

The inclusion proviso in this assessment for arthroplasty is that at least one of the three compartmental joints should be a Kellgren & Lawrence (K&L) grade 4 and have bone-on-bone contact. This can be assessed by standing knee Anteroposterior (AP), lateral, “Stress-view” in 20° flexion and 15° (medial) or 45° (lateral) PA (Rosenberg View). The “unaffected” (healthy) weight-bearing TF compartment must be more than 5mm in width and parallel (congruent) to the opposing joint surfaces on the stress view.

NB: When the stress view demonstrates a “wedge deformity” in the “healthy” compartment, it indicates ligamentous instability or more wear than anticipated in the healthy compartment.

The PFJ assessment uses the 30° skyline and lateral views, as well as the stress and Rosenberg views to confirm the integrity of the weight-bearing compartments when considering Patellofemoral Arthroplasty (PFA).

The grading system consists of four grades with treatment options to be implemented.
Despite the specific grading achieved, clinical reasons can influence the final decision for a particular prosthesis, for example rheumatoid arthritis (RA), clinically assessed disruption of ligaments, excessive deformities or patient-specific considerations or preferences. KOGS cannot be the only deciding factor.

This system is based on wear patterns and constitutional wear progression.

GRADE 1: UKA OR PFA
SINGLE COMPARTMENT OSTEOARTHRITIS (OA) / SINGLE K&L GRADE 4 (SINGLE TF or PFJ)

Degeneration affects any one of the three compartments, with bone-on-bone degeneration.
Osteophytes may be present in other compartments but these retain their integrity, being >5mm with a parallel joint surface required in the weight-bearing compartment.
The KOGS Grade 1 can have OA changes in the other compartments e.g. the PFJ might have OA changes but is not grooved or dysplastic and therefore remains a Grade 1 or osteophytes not affecting joint space in the ipsilateral joint remains a Grade 1.

e.g. A TF joint with bone-on-bone degeneration or less than 5mm joint space and associated, early (non-symptomatic) PFJ OA (not dysplastic or grooved)
OR
Osteophytes in TF’s without narrowing (Full thickness cartilage as seen on Stress-view or 15° and 45° Rosenberg View) and grooving of the PFJ (isolated PFJ OA).

GRADE 2: UKA
SINGLE TF COMPARTMENT / SINGLE TF K&L GRADE 4 WITH “PSEUDOLAXITY” – SUBLUXATION ON AP KNEE VIEW
(SINGLE TF + SUBLUX REDUCTION)

The lesion is isolated to the TF joints but with large lesions causing subluxation (due to the defects) on the AP views. However this is reduced by the stress views and the ligaments are stable. The healthy ipsilateral compartment is maintained (Parallel and > 5mm) despite the presence of osteophytes.
Evaluation of subluxation is dependent on good Stress and Rosenberg views with the identification seen where the tibial spines move towards the opposing lateral femoral condyle or seen with the lateral femur displaced medially to the lateral tibia. Lateral subluxation is possible but seldom seen.

GRADE 3: UKA + ACL OR UKA + PFA OR TKA
TWO PATHOLOGIES THAT INVOLVE ONE TF JOINT AND A CONCOMITANT SECOND PROBLEM

3 A: SINGLE TF COMPARTMENT / SINGLE TF K&L GRADE 4 AND CONCOMITANT ANTERIOR CRUCIATE LIGAMENT (ACL) INSTABILITY

Single TF bone-on-bone (K&L grade 4) with an ACL instability or previous surgery to the ACL. (UKA + ACL or TKA)

3 B: SINGLE TF COMPARTMENT / SINGLE TF K&L GRADE 4 AND CONCOMITANT GROOVED PFJ OA

A single TF OA with a grooved PFJ is involved with less than 5mm joint space (Osteophytes only at the PFJ is not sufficient and does not qualify as a Grade 3 B but rather a Grade 1). A previous Patellectomy is also considered to be “pathology” and therefore a Grade 3 B when associated with a TF bone-on-bone (K&L grade 4). (UKA + PFA or TKA)

GRADE 4: BI-CONDYLAR UKA OR BI-CRUCIATE RETAINING TKA OR TKA
BOTH TF COMPARTMENTS

4 A: Both TFJ’s are affected with ligament stability. (BI-CONDYLAR UKA or BI-CRUCIATE RETAINING TKA)
4 B: Both TFJ’s wear and have ligament instability. (TKA)

These treatment recommendations depend on clinical evaluation and patient compliance with Grade 1 PFJ possibly requiring a TKA and Grade 3 knees similarly a TKA.