Updated: 9/8/2020

Unicompartmental Knee Replacement

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Introduction
  • Surgical option for knee arthritis when only one compartment of the knee is involved  
  • Epidemiology
    • 5% of surgeries where knee arthroplasty is indicated are unicompartmental knee replacements
    • location
      • medial compartment is most common
  • Types of implants
    • fixed-bearing
      • historical standard of care
    • mobile-bearing
      • pros
        • weightbearing through the meniscus increases conformity and contact without increasing constraint
        • decrease in wear pattern
        • excellent survivorship out to the second decade
      • cons
        • technically demanding
        • bearings can dislocate
  • Advantages
    • compared to TKA
      • faster rehabilitation and quicker recovery
      • less blood loss
      • less morbidity
      • less expensive
      • preservation of normal kinematics
        • theory is that retaining ACL, PCL and other compartments leads to more normal knee kinematics
      • smaller incision
        • less post-operative pain leading to shorter hospital stays
    • compared to osteotomy
      • faster rehabilitation and quicker recovery
      • improved cosmesis
      • higher initial success rate
      • fewer short-term complications
      • lasts longer
      • easier to convert to a TKA
Indications
  • Indications
    • controversial and vary widely
    • as an alternative to total knee arthroplasty or osteotomy for unicompartmental disease
    • classicaly reserved for older (>60), lower-demand, and thin (<82 kg) patients 
      • 6% of patient's meet the above criteria with no contraindications
    • new effort to expand indications to include younger patients and patients with more moderate arthrosis
  • Contraindications
    • inflammatory arthritis  
    • ACL deficiency
      • absolute contraindication for mobile-bearing UKA and lateral UKA 
      • controversial for medial fixed-bearing
    • fixed varus deformity > 10 degrees 
    • fixed valgus deformity >5 degrees
    • restricted motion
      • arc of motion < 90°
      • flexion contracture of > 5-10°
    • previous meniscectomy in other compartment
    • tricompartmental arthritis (diffuse or global pain)
    • younger high activity patients and heavy laborers
    • overweight patients (> 82 kg)
    • grade IV patellofemoral chondrosis (anterior knee pain)  
Technique
  • Procedural tips
    • avoid overcorrections
      • undercorrect the mechanical axis by 2-3 degrees
      • overcorrection places excess load on unresurfaced compartment
    • remove osteophytes (peripheral and notch)
    • resect minimal bone
    • avoid extensive releases
    • avoid edge loading
    • prevent tibial spine impingement with proper mediolateral placement
    • avoid making a varus tibial cut which increases the chance for loosening
    • use caution when placing the proximal tibial guide pins to avoid stress fractures
    • correct varus deformity to 1-5 degrees of valgus
Complications
  • Aseptic loosening  
    • most common cause of early failure (5 years) at somewhere between 25%-45.3% 
  • Stress fractures  
    • always involve tibia
    • associated with high activity and patient weight
    • clinically there will be a pain free interval followed by spontaneous pain with activity
    • blood commonly found on joint aspiration
    • risk factors 
      • penetrating posterior tibial cortex with guide pin, placing guide pin medial in periphery, re-drilling for guide pin, and under-sized tibial component
  • Intra-operative fractures 
    • associated with forceful impacting of implant
Outcomes
  • Fixed-bearing
    • 1st decade results
      • 10-year survivorship from studies done in 1980s and 1990s ranges from 87.4% to 96%
      • the standard faliure rate in the first decade is 1%
    • 2nd decade results
      • rapid decline in survivorship ranging from 79% to 90%
  • Mobile-bearing
    • excellent clinical results with 15-year survivorship reported at 93%
  • Long-term results
    • lateral compartment arthroplasties have equivalent results to medial 
    • revision rates are worse than total knee revision rates 
    • Patellofemoral arthroplasty (PFA) has good outcomes for isolated patellofemoral arthritis 
      • Previous generation designs (i.e. inlay style) exhibited high rates of patellar instability 
      • Newer generation designs (i.e. outlay style) replaces entire anterior trochlear surface and minimizes risk of patellar instability
      • Long term mode of failure remains progression of tibiofemoral arthritis 
    • causes of late failure (>5 years)
      • progress of osteoarthritis (idiopathic, over-correction, more common with mobile-bearing)  
      • component failure (overload due to under-correction)
      • component loosening (common in fixed-bearing)
      • patella impingement on femoral component (patella pain)
      • polyethylene wear
 
 

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(OBQ09.60) A 61-year-old male had a minimally-invasive unicompartmental knee replacement 8 months ago. He did well until recently when he developed persistent right knee pain that is worse with weight bearing. He denies any fevers or recent trauma. He does report that he had been exercising more over the past few months in an attempt to lose weight. WBC, ESR and C-reactive protein levels are normal. An AP radiograph and bone scan are shown in Figure A and B. What is the most likely cause of his symptoms? Tested Concept

QID: 2873
FIGURES:
1

Component failure/ polyethylene failure

8%

(191/2413)

2

Infection

1%

(18/2413)

3

Pes anserine bursitis

3%

(84/2413)

4

Stress fracture

87%

(2103/2413)

5

Complex regional pain syndrome

0%

(8/2413)

L 1 C

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(OBQ09.18) All of the following are contraindications to medial unicondylar knee arthroplasty EXCEPT: Tested Concept

QID: 2831
1

Flexion contracture greater than 10 degrees

4%

(117/3272)

2

Varus deformity greater than 10 degrees not correctable with stress testing

4%

(126/3272)

3

Lateral knee joint line pain

12%

(406/3272)

4

Rheumatoid arthritis

6%

(212/3272)

5

Osteonecrosis of the medial femoral condyle

73%

(2394/3272)

L 2 C

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(OBQ08.255) A 40-year-old man has moderate lateral compartment arthritis several years after undergoing a partial lateral meniscectomy. He has a correctable 5 degree valgus knee deformity compared to his other limb. His patellofemoral and medial compartments do not show any radiographic signs of degenerative changes. His knee has full range of motion and is stable on exam. After failing nonoperative treatments, which surgical option is most likely to give him the best outcome? Tested Concept

QID: 641
1

Valgus producing high tibial osteotomy

3%

(105/3262)

2

Varus producing distal femoral osteotomy

85%

(2783/3262)

3

Total knee replacement

8%

(253/3262)

4

Arthroscopic debridement and chondroplasty

3%

(91/3262)

5

Tibial tubercle osteotomy with anteromedialization

0%

(15/3262)

L 2 C

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(OBQ07.20) All of the following are contraindications for a medial unicompartmental knee replacement EXCEPT? Tested Concept

QID: 681
1

ACL deficiency

3%

(54/2018)

2

Anterior compartment osteophytes and pain beneath the patella

4%

(81/2018)

3

Medial sided knee pain with medial and lateral compartment osteophytes and 3 degrees of varus deformity

11%

(214/2018)

4

Lateral sided knee pain with degenerative changes isolated to the medial compartment and 7 degrees of varus deformity

10%

(195/2018)

5

Flexion contracture of 3 degrees

73%

(1469/2018)

L 2 C

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(OBQ06.107) Which of the following benefits can be expected from unicompartmental knee arthroplasty compared to total knee arthroplasty for medial compartment knee arthritis? Tested Concept

QID: 293
1

Better clinical outcomes at one year follow-up.

2%

(44/2019)

2

Greater survivorship rate at 10 year follow-up

1%

(16/2019)

3

Faster postoperative rehabilitation

94%

(1900/2019)

4

Better postoperative knee alignment

2%

(34/2019)

5

Reduced risk of secondary surgery within the first year

1%

(15/2019)

L 1 C

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(OBQ05.26) When performing a unicondylar knee replacement, a smaller incision without dislocation of the patella offers what advantage over a standard, patella-everting approach? Tested Concept

QID: 63
1

the option to convert to a total knee arthroplasty if needed

2%

(20/1199)

2

more anatomic positioning of the components

4%

(46/1199)

3

better ultimate range-of-motion

2%

(28/1199)

4

increased 10-year implant survival rate

1%

(6/1199)

5

improved rate of recovery

91%

(1094/1199)

L 1 D

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(OBQ04.259) A 60-year-old male tennis player undergoes a unicompartmental knee arthroplasty (UKA) shown in Figures A and B. Which of the following statements regarding this procedure is true? Tested Concept

QID: 1364
FIGURES:
1

Compared to total knee arthroplasty (TKA), UKA more closely approximates native knee kinematics

86%

(2595/3001)

2

Patients undergoing a UKA and TKA have equivalent blood loss and pain medication requirements

1%

(44/3001)

3

Compared to their TKA counterparts, UKA patients have a slower return to function

1%

(20/3001)

4

There is no difference in range of motion at short or long term follow-up when compared with TKA

9%

(264/3001)

5

Postoperative hospital stay is equivalent for UKA and TKA patients

2%

(66/3001)

L 1 C

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Evidence (54)
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CASES (7)
Topic COMMENTS (34)
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