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- Discussion:
   - 2 subtypes: rim avulsions and rim compression frx;
   - all knees w/ rim frxs have capsular and collateral ligament injuries;
   - avulsions of rim of tibial plateau articular surface or compression frx of that rim almost always are accompanied by significant collateral or cruciate ligament injury;
   - may represent fracture dislocation;

- Radiographs:
   - diagnostic studies of these fractures should include x-rays obtained w/ knee in genu valgus and genu varus stress;
   - diagnosis ismade on anteroposterior x-rays that show a rim compression frx and/or a rim avulsion fracture;

- Rim Avulsion Fracture: 
    - comprimising 16% of frx dislocations of the Fracture Dislocations;
    - occurs w/ severe valgus or varus forces exerted on joint capsule & collateral ligaments creating ligament injury along w/ fragment of articular margin;
    - this type almost exculsively involves lateral tibial plateau, w/ avulsion frags of capsule attachment, Gerdy's tubercle, or plateau;
    - disruption of either or both cruciate ligaments is common;
    - menical injury is rare;
    - neurovascular injuries occur in 30% of frxs, & nearly all type III frx are unstable;
    - lateral approach allows screw fixation of the articular rim and repair of avulsed iliotibial band & collateral ligaments;

 - Rim Compression Fracture:
    - this injury accounts for about 12% of all fracture dislocations; 
    - it is almost always unstable;
    - opposite collateral ligament complex & cruciate ligaments are usually avulsed or torn, allowing the tibia to sublux which causes a secondary compression frx
             of a portion of anterior, posterior, or middle articular rim;
 
- Non Operative Treatment: 
       - indicated if rim compression is small & knee shows < 10 deg of instability; 
       - frx brace, cast brace, or long leg cast provides sufficient support during the 6 to 8 weeks required for soft tissue healing;

- Operative Treatment: 
       - PreOp Planning    
       - is used in knees w/ > 10 deg of instability with avulsion frxs, or with large rim compression fractures;
       - parapatellar approach allows debridement of small fragments, elevation and stabilization of the larger fragments, & repair of cruciate and opposite collateral ligaments;
       - rim compression frxs are elevated thru smal incision made just below joint line;
       - articular surface and the fracture are visualized thru the infra-meniscal interval;
       - small coritcal window is made distal to the rim fracture & depressed articular margin is elevate to normal position w/ osteotome or elevator;
       - local cancellous grafts are used to fill the resulting defect;
       - if there is rim avulsion frx on opposite side of knee, it is repaired and fixed in its bed by a screw;
 
- Post Operative Care and Compications:
       - cast brace is used for 6 weeks;
       - wt bearing is delayed for 8 weeks to allow consolidation of frx and healing of repaired capsular & collateral ligaments;
       - loss of screw or pin fixation is main complication of rim frx;
       - late problem noted most often is knee instability