1. Types of Patellar Instability
PRiSM
Patellar Instibialiy RIG
Jason Rhodes, MD, MS Alex Tagawa, BA

2. Purpose/Background
Purpose: Description of Types and Classification of Patellar Instability
No Legitimate Accepted Classification
Etiologies affect treatment and long term outcomes
2/3 of acute patellar dislocations are in patients less than 20 years of age2

3. Types Lateral, Medial, Vertical, Rotational
First-time vs Recurrent LPD
Congenital Dislocation
Developmental Dislocation
Habitual Dislocation
Permanent Dislocation
Chotel, et al.14

4. Location of Dislocation
Lateral
Most common
23.2 per 100,000 person years
Age and sex adjusted3
Medial
Less Common
Vertical
Superior or inferior to trochlea
Rotational
Axial dislocation

5. Nomenclature for Lateral Instability
Parikh SN, Lykissas MG17

6. Recurrence Primary acute dislocations Recurrence
77.4 (95% CI: ) per 100,000 persons per year in males4
Recurrence
22.7% (95% CI ) of suffering a recurrent dislocation, with young girls aged experiencing the highest risk5

7. Congenital Patellar Dislocation
Rare condition in which the patella is permanently dislocated and cannot be reduced manually6
Presents immediately after birth6
Flexion contracture of the knee, genu valgus, external tibial torsion and foot deformity6
Can be associated with other diagnoses such as arthrogryposis7

8. Developmental Patellar Dislocation
Not present at birth but develops after walking age17
40% of immature patients may develop recurrent instability8,9
10% of these patients may develop contralateral knee patellar instability8,10

9. Habitual Dislocation Habitual dislocation in knee flexion8
At variable angles of flexion8
Patella reduces in extension8
Habitual dislocation in knee extension8
Patella slides over the lateral trochlea when knee reaches almost full extension8

10. Permanent Dislocation
After walking age
Distinguished morphologically by normal patellar height and an intact, rounded lateral trochlea and single-facet patella
Antalgic gait common8

11. Classification Classification vs. Types
No official classification
Many different ways to classify
Etiology
Anatomically
Underlying pathologies
Age
Pathophysiology

12. Historical/Literature Review
Adult population
Dejour, et al.11
Garin, et al.12
Sillanpaa13
Pediatric Population
Chotel, et al.14
Hiemstra. et al.15
Frosch and Schmeling16
Parikh and Lykissas17
Weeks, et al. (Green)18
Keyes, et al. (Parikh) AJO Summary1

13. Dejour, et al.11 Anatomic abnormalities and pain 3 types Major
More than 1 documented dislocation
Objective
1 dislocation and anatomic abnormality
Potential
Radiographic abnormalities and patellar pain
Jaquith, Parikh19

14. Garin, et al.12 2 groups Major Recurrent
Permanent or habitual
Recurrent
Reported that soft tissue ligamentous transfer was a successful method for patellar realignment but pending the underlying cause often complementary procedures were needed as well

15. Sillanpaa13
1st time
Recurrent
Acute injuries only

16. Pediatric Dislocations1
Historical Classifications
Traumatic
Congenital

17. Chotel, et al.14 5 categories Congenital Permanent
At birth with a functional genu valgum
Permanent
1-5 years of age
Habitual during knee flexion
5-8 years of age
Habitual during extension
Recurrent
Preadolescence or adolescence due to atraumatic event

18. Chotel, et al.14

19. Hiemstra. et al.15 WARPS STAID
Weak, atraumatic, risky anatomy, pain and subluxation
Continued instability
Valgus, ligamentous laxity, rotational abnormalities, shallow and short trochlear groove, and patella alta
STAID
Strong, traumatic, anatomy normal, instability and dislocation
No underlying anatomic issues

20. Frosch and Schmeling16 Type 1 Type 2 Type 3 Type 4 Type 5
Simple dislocation with no maltracking or instability
Type 2
High risk of redislocation and no maltracking
Type 3
5 subcategories
Contracture, patella alta, pathological tibial tuberosity and TG distance
Type 4
Highly unstable “floating patella”
Type 5
Patellar maltracking without instability

21. Frosch and Schmeling16

22. Parikh and Lykissas17 Type 1 Type 2 First-time dislocation
Recurrent instability
2A – positive apprehension
2B – instability related to anatomic abnormalities

23. Parikh SN, Lykissas MG17

24. Weeks, et al. (Green)18 Traumatic Obligatory Fixed laterally
Acute or recurrent
Obligatory
Flexion or extension
Fixed laterally
Often congenital abnormalities
All can be syndromic
Skeletal dysplasia, Ehlers-Danlos, CP, Marfan disease, nail-patella syndrome, Down Syndrome, Rubenstein-Taybi syndrome, Kabuki syndrome

25. Keyes, et al

26. Patellar Dislocation Types/Classifications
No one excepted standard classification
Many different patterns of dislocation/instability
Many underlying etiologies and pathologies
Appropriate to evaluate and address all issues
Where do we start?

27. References
Keyes S, Price M, Green DW, Parikh SN, Special considerations for pediatric patellar instability, Am J Orthop (Belle Mead NJ) Mar;47(3). doi: /ajo
Duthon VB, Acute traumatic patellar dislocation, Orthop Traumatol Surg Res Feb;101(1 Suppl):S doi: /j.otsr , Epub 2015 Jan 12.
Sanders TL, Pareek A, Hewett TE, Stuart MJ, Dahm DL, Krych AJ, Incidence of First-Time Lateral Patellar Dislocation: A 21-Year Population-Based Study, Sports Health, 2018 Mar/Apr;10(2): doi: / Epub 2017 Aug 10.
Sillanpää P, Mattila VM, Iivonen T, Visuri T, Pihlajamäki H, Incidence and risk factors of acute traumatic primary patellar dislocation, Med Sci Sports Exerc Apr;40(4): doi: /MSS.0b013e f.
Gravesen KS, Kallemose T, Blønd L, Troelsen A, Barfod K, High incidence of acute and recurrent patellar dislocations: a retrospective nationwide epidemiological study involving primary dislocations, Knee Surg Sports Traumatol Arthrosc, 2018 Apr;26(4): doi: /s Epub 2017 Jun 23.
Wada A, Fujii T, Takamura K, Yanagida H, Surijamorn P, Congenital dislocation of the patella, J Child Orthop Mar;2(2): doi: /s Epub 2008 Mar 4.
Eilert RE, Congenital dislocation of the patella, Clin Orthop Relat Res Aug;(389):22-9.
Popkin CA, Bayomy AF, Trupia EP, Chan CM, & Redler LH, (2018). Patellar Instability in the Skeletally Immature, Current Reviews in Musculoskeletal Medicine, 11(2), 172–181.
Lewallen LW, McIntosh AL, Dahm DL, Predictors of recurrent instability after acute patellofemoral dislocation in pediatric and adolescent patients, Am J Sports Med, 2013;41(3):575–581. doi: /

28. References Continued
Sanders TL, Pareek A, Hewett TE, Stuart MJ, Dahm DL, Krych AJ, High rate of recurrent patellar dislocation in skeletally immature patients: a long-term population-based study, Knee Surg Sports Traumatol Arthrosc
Dejour H, Walch G, Nove-Josserand L, Guier C, Factors of patellar instability: an anatomicradiographic study, Knee Surg Sports Traumatol Arthrosc. 1994;2(1):19-26.
Garin C, Chaker M, Dohin B, Kohler R, Permanent, habitual dislocation and recurrent dislocation of the patella in children: surgical management by patellar ligamentous transfer in 50 knees, Rev Chir Orthop Reparatrice Appar Mot. 2007;93(7):
Sillanpaa P, Terminology of patellar dislocation, In: Sillanpaa P, ed. Trauma. Saarbrucken, Germany: Lambert Academic Publishing; 2010:16-18.
Chotel F, Bérard J, Raux S, Patellar instability in children and adolescents, Orthop Traumatol Surg Res. 2014;100(suppl 1): doi: /j.otsr
Hiemstra LA, Kerslake S, Lafave M, Heard SM, Buchko GML, Introduction of a classification system for patients with patellofemoral instability (WARPS and STAID), Knee Surg Sport Traumatol Arthrosc. 2014;22(11): , doi: /s
Frosch KH, Schmeling A, A new classification system of patellar instability and patellar maltracking. Arch Orthop Trauma Surg, 2016;136(4): doi: /s
Parikh SN, Lykissas MG, Classification of lateral patellar instability in children and adolescents, Orthop Clin North Am. 2016;47(1): doi: /j.ocl
Weeks KD, Fabricant PD, Ladenhauf HN, Green DW, Surgical options for patellar stabilization in the skeletally immature patient, Sports Med Arthrosc Rev. 2012;20(3):
Jaquith BP, Parikh SN, Predictors of Recurrent Patellar Instability in Children and Adolescents After First-time Dislocation, J Pediatr Orthop Oct/Nov;37(7): doi: /BPO