A Case of Subchondral Insufficiency Fracture of the Knee at Lateral Femoral Condyle Treated With Unicompartmental Knee Arthroplasty

Abstract

Spontaneous insufficiency fracture of the knee is a potentially devastating yet poorly understood disease entity that can lead to secondary osteoarthritis. Most cases involve the medial femoral condyle, and the lateral femoral condyle is rarely affected. The optimal treatment for spontaneous insufficiency fracture of the lateral femoral condyle remains undetermined, and there are no previous dedicated reports on treatment outcome with unicompartmental knee arthroplasty. A middle-aged lady presented with subacute left knee pain and a locked knee. Subsequent imaging studies revealed a spontaneous insufficiency fracture of the lateral femoral condyle. In view of the isolated compartment involvement, unicompartmental knee arthroplasty was performed with satisfactory outcome. At 1 year postoperatively, the patient had complete resolution of knee pain and was able to resume working.

Full text

Case Report
A Case of Subchondral Insufficiency Fracture of the Knee at Lateral
Femoral Condyle Treated With Unicompartmental Knee Arthroplasty
Chun Hin Lo, MBBS(HKU)
*
,
Yan Ho Bruce Tang, MBBS(HKU), MRCSEd, FHKCOS, FRCSEd(Orth), FHKAM(Orthopaedic
Surgery)
Department of Orthopaedics and Traumatology, Tuen Mun Hospital, Tuen Mun, New Territories, Hong Kong
article info
Article history:
Received 9 February 2022
Received in revised form
28 March 2022
Accepted 3 April 2022
Available online xxx
Keywords:
Spontaneous insufficiency fracture of the
knee
Unicompartmental knee arthroplasty
abstract
Spontaneous insufficiency fracture of the knee is a potentially devastating yet poorly understood disease
entity that can lead to secondary osteoarthritis. Most cases involve the medial femoral condyle, and the
lateral femoral condyle is rarely affected. The optimal treatment for spontaneous insufficiency fracture of
the lateral femoral condyle remains undetermined, and there are no previous dedicated reports on
treatment outcome with unicompartmental knee arthroplasty. A middle-aged lady presented with
subacute left knee pain and a locked knee. Subsequent imaging studies revealed a spontaneous insuf-
ficiency fracture of the lateral femoral condyle. In view of the isolated compartment involvement, uni-
compartmental knee arthroplasty was performed with satisfactory outcome. At 1 year postoperatively,
the patient had complete resolution of knee pain and was able to resume working.
©2022 The Authors. Published by Elsevier Inc. on behalf of The American Association of Hip and Knee
Surgeons. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
licenses/by-nc-nd/4.0/).
Introduction
Spontaneous insufficiency fracture of the knee (SIFK) was initially
termed spontaneous osteonecrosis of the knee (SONK) and was first
described by Ahlb€
ack et al. in 1968, where a group of patients with
atraumatic severe knee pain were found to have primary osteonec-
rosis [1]. The disease was later classified into 3 different entities,
namely spontaneous osteonecrosis, secondary osteonecrosis, and
postarthroscopic osteonecrosis [2]. A latter study by Yamamoto et al.
showedthat the primary event is a subchondral insufficiency fracture,
followed by secondary localized osteonecrosis, suggesting SONK was
indeed a misnomer [3]. SIFK predominantly affects the medial
femoral condyle, where lateral femoral condyle involvement is much
less common [4]. The patients typically present with acute kneepain,
which is exaggerated by mechanical load and relieved with rest. The
pain frequently worsens at night. Physical examination is usually
unremarkable except localized tenderness over the affected area.
Conventionally, total knee arthroplasty (TKA) has been the final
treatment for patients with a poor response to conservative treat-
ment. However, as SIFK usually involves a single compartment, uni-
compartmental knee arthroplasty (UKA) is an appealing alternative
with advantages in preservation of native joint kinematics and pro-
prioception, particularly among younger patients [5].
Case history
A 66-year-old Chinese female cleaning worker, with a past
history of hypertension and impaired fasting glucose, first pre-
sented to the emergency department with left knee pain and
swelling for 1 month. Radiographs of the knee was performed, but
the patient was only reported to have degenerative joint disease
(Fig. 1a). She experienced persistent left knee pain afterwards and
was only able to ambulate with a walking stick.
Ethics approval was obtained from the New Territories West Cluster Research
Ethics Committee of Hospital Authority, Hong Kong.
The clinical data used during the current study are available from the corre-
sponding author on reasonable request.
Funding: This study did not receive any funding.
Authors' contribution: C.H.L. collected clinical data and prepared the manuscript. Y.
H.B.T. recruited the patient, performed the operation, and provided supervision in
writing the manuscript. All authors read and approved the final manuscript.
*Corresponding author. Department of Orthopaedics and Traumatology, Tuen
Mun Hospital, Tuen Mun, New Territories, Hong Kong. Tel.: þ852 2468 5111.
E-mail address: chivanlo.hk@gmail.com
Contents lists available at ScienceDirect
Arthroplasty Today
journal homepage: http://www.arthroplastytoday.org/
https://doi.org/10.1016/j.artd.2022.04.002
2352-3441/©2022 The Authors. Published by Elsevier Inc. on behalf of The American Association of Hip and Knee Surgeons. This is an open access article under the CC BY-
NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Arthroplasty Today 16 (2022) 15e20

At 1 month after the initial emergency department presenta-
tion, the patient returned to the hospital again and was admitted to
the orthopaedics and traumatology unit. Upon admission, the pa-
tient was found to have left knee tenderness, predominantly over
the lateral joint line. She presented with a locked knee with the
range of movement limited at 30 to 130 degrees. The patient’s pain
was exacerbated with knee extension.
Radiograph of the left knee was repeated upon admission.
Further progression of osteonecrosis at the lateral femoral condyle
with a displaced curvilinear bony lesion was noted (Fig. 1b). Mag-
netic resonance imaging (MRI) scan revealed a displaced osteo-
chondral lesion with secondary degenerative changes shown as a
cortical defect at the inferior border of the lateral femoral condyle
with extensive marrow edema. There was a displaced lateral
meniscal tear as evidenced by the loss of a normal anterior horn
and anterior body of the lateral meniscus (Fig. 2a-c). A subsequent
plain computed tomography scan revealed an isolated subchondral
bone defect at the lateral femoral condyle with a displaced intra-
articular radiopaque curvilinear fragment (Fig. 3a-c). Overall fea-
tures were suggestive of SIFK of the lateral femoral condyle within
the left knee.
At 2 weeks after the hospital admission, a left UKA for the
lateral compartment was performed. A lateral parapatellar
approach was used. Intraoperative joint inspection confirmed
SIFK of the lateral femoral condyle. There was a 1.5 5-cm
subchondral defect at the weight-bearing surface of the lateral
femoral condyle. A 1.5 5-cm intra-articular dislodged cartilage
fragment was retrieved (Fig. 4a and b). There were only small
areas of partial cartilage loss over the medial femoral condyle
and patellofemoral joint. The anterior and posterior cruciate
ligaments were intact.
The lateral UKA was performed using the Zimmer®Uni-
compartmental High Flex Knee system (ZUK; Zimmer, Warsaw,
IN). Soft-tissue dissection was limited to what was necessary for
exposure only. We adopted the spacer block technique and
resected the tibia first. The resection was ensured to be minimal
and orthogonal, reproducing the native slope with about 10

internal rotation. The distal femoral cut was guided by the spacer
block in extension. Despite the large central defect at the lateral
femoral condyle, we could still identify the correct extension gap
with the remaining bone. The lesion was resected during the
distal femur cut (Fig. 4c), and the underlying bone appeared
healthy. The remaining femoral cuts were completed, ensuring
correct rotation and no oversizing of the prosthesis. The implants
were fixed with cement and taken into account of the “screw-
home”mechanism, with the tibial component placed at 10

in-
ternal rotation and the femoral component placed as lateral as
possible. The thinnest available polyethylene insert was used.
There was no overtension or impingement throughout the range
of motion (Fig. 4d).
Figure 1. Preoperative radiograph. (a) Radiograph of left knee taken upon presenting to the emergency department showed an area of osteonecrosis over the lateral femoral
condyle on the anteroposterior view with an associated curvilinear bony lesion on the lateral view. (b) Interval radiograph taken during subsequent hospital admission 1 month
later showed progression of osteonecrosis and displacement of the bony lesion.
Figure 2. Preoperative MRI. (a) Coronal proton-density-weighted fat-suppressed and (b) T1-weighted MRI images of left knee demonstrated subchondral insufficiency fracture at
the lateral femoral condyle with a curvilinear hypodensity parallel to the femoral condyle subchondral bone plate. There was surrounding bone marrow edema-like signals at the
lateral femoral condyle. There was extrusion of the lateral meniscus. (c) Sagittal proton-density-weighted fat-suppressed sequence showed a displaced osteochondral lesion.
C.H. Lo, Y.H.B. Tang / Arthroplasty Today 16 (2022) 15e2016

Postoperative radiographs of the knee and lower limb scano-
gram showed satisfactory alignment (Fig. 5a-c). The patient had an
uneventful recovery and was self-ambulatory. A course of physio-
therapy was completed with progressive improvement in knee
mobility and muscle strength. The patient was highly satisfied with
the outcome. Upon the latest follow-up at 1 year postoperatively,
the patient experienced complete resolution of knee pain. The
active range of movement of the knee was at 0-130 degrees, with
extensor and flexor muscle power grade 5 on the Oxford Scale. The
patient was able to walk unaided and resume her job as a cleaning
worker. The preoperative Knee Society Knee Score was 32 points,
and the preoperative Knee Society Function Score was 15 points.
The postoperative 1-year Knee Society Knee Score and Knee Society
Function Score improved to 99 points and 90 points, respectively. A
written informed consent of participation in publication was ob-
tained from the patient.
Discussion
In the initial description by Ahlb€
ack et al., a cohort of pa-
tients with knee osteonecrosis in the absence of other known
clinical entities were first labelled as having SONK [1]. Lotke
Figure 3. Preoperative CT. (a) Axial and (b) coronal plain CT images of left knee demonstrated flattening of the lateral femoral condyle and a subchondral lesion. There was
associated osteoarthritic changes at the lateral compartment of the femorotibial joint, while other compartments remained relatively unaffected. (c) Sagittal images showed a
displaced intra-articular radiopaque curvilinear fragment. CT, computed tomography.
Figure 4. Intraoperative photos. (a) A 1.5 5-cm subchondral defect was found at the weight-bearing surface of the lateral femoral condyle. (b) A 1.5 5-cm intra-articular
dislodged cartilage fragment was retrieved. (c) From left to right: the resected SIFK lesion during distal femur cut, the dislodged cartilage fragment, and the tibia cut. (d)
Lateral UKA was performed.
C.H. Lo, Y.H.B. Tang / Arthroplasty Today 16 (2022) 15e20 17

and Ecker later proposed that microfractures in osteoporotic
subchondral bone was a possible etiological mechanism [6].
Joint fluid could flow into the bone through broken articular
cartilage, resulting in bone marrow edema, focal ischemia, and
subsequent osteonecrosis [6]. Akamatsu et al. showed a positive
correlation between low bone mineral density and the inci-
dence of SONK among women older than 60 years [7]. Yama-
moto and Bullough demonstrated the primary event of SONK
was subchondral insufficiency fracture followed by secondary
osteonecrosis between the fracture line and the subchondral
bone plate, thus suggesting the shift of terminology into SIFK
[3]. Historically, both SONK and SIFK have been used to describe
thesamedisease,andthe2termsweresometimesused
interchangeably in the literature [8]. Some studies have chal-
lenged the term “spontaneous”in SONK does not reflect the
actual pathophysiology and should be considered as a
misnomer. The current literature generally supports that SONK
is the end result of subchondral fracture and part of the SIFK
disease spectrum [9].
In a review by Sibilska et al., the prevalence of SIFK is reported at
3.4% in elderly patients [10]. The major risk factors include
advanced age, female sex, low bone mineral density, cartilage
degeneration, and meniscus extrusion. The condition has a pre-
dominantly single compartment involvement in the knee joint. The
medial femoral condyle is affected in up to 94% of cases, while
lateral femoral condyle involvement is much rarer. A review by
Pareek et al. showed the lateral femoral condyle was involved in
7.2% of SIFK cases [8]. Some studies attributed the difference in
blood supply between the medial and lateral femoral condyles.
From cadaveric studies, the medial femoral condyle has only
intraosseous blood supply with watershed areas, whereas the
lateral femoral condyle has rich intraosseous and extraosseous
blood supply, making it less susceptible to osteonecrosis from bone
ischemia. Clinically, patients with SIFK of the lateral femoral
condyle may also have an atypical disease course. In a case series of
11 patients by Ohdera et al., the patients with lateral SIFK may have
symptom onset in the middle age without osteoporotic bones. The
knee pain is not usually abrupt nor worse at night, and the lower
extremity is not always valgus-aligned, setting apart from the
clinical features of medial SIFK [11]. Nonetheless, due to the scarcity
of reported cases, the precise natural course of disease remains
unclear.
Figure 5. Postoperative radiograph. (a) Anteroposterior and (b) lateral radiographs of left knee taken at 6 months postoperatively, and (c) scanogram of lower limbs taken on
postoperative day 5, showing in situ UKA implants.
C.H. Lo, Y.H.B. Tang / Arthroplasty Today 16 (2022) 15e2018

Regarding radiological investigations, the radiographs usually
show no characteristic findings at early stages of the disease.
Distinct radiolucent areas at the lateral femoral condyle may be
observed in advanced cases with subchondral collapse. The im-
aging modality of choice is MRI with T2-weighted and proton-
density-weighted sequences. The characteristic findings of SIFK
include a hypointense line in the subarticular bone marrow
representing a subchondral fracture and focal depression of the
subchondral bone plate. There may be a fluid-filled cleft under-
lying the subchondral bone plate, indicating gross collapse and
separation. The surrounding bone edema often involving the
entire femoral condyle is in contrast to the more localized edema
adjacent to cartilage loss observed in cases of osteoarthritis [12].
The treatment of SIFK can be nonoperative or operative
depending on the size and radiological staging of the lesion. The
Koshino classification was first described in 1979, where small
radiolucent lesions measuring <3.5 cm
2
tend to regress with con-
servative treatment, while large lesions measuring >5cm
2
are
more likely to progress into subchondral collapse [4]. Another
study by Lotke et al utilized the size of a lesion as a percentage of
the affected femoral condyle, where lesions involving more than
50% of the condylar area would quickly progress to collapse
requiring arthroplasty [6].
The options of nonoperative treatment for early-stage SIFK
include physiotherapy, analgesics, nonsteroidal anti-inflammatory
drugs, and bisphosphonates [13e15]. In a case series of early-
stage SIFK patients treated with nonoperative management, all
cases had resolution of symptoms and MRI findings within 6
months [13]. For patients with larger SIFK lesions, or failed
nonoperative treatment, surgical interventions are indicated.
Several joint-preserving techniques have been described, including
arthroscopic debridement, osteochondral graft, and high tibial
osteotomy. Although the results from some trials demonstrated
favorable outcomes in delaying the need of arthroplasty, the lack of
high-quality evidence precludes widespread adoption of these in-
terventions [16e19].
For the patients with advanced disease who failed conservative
management, TKA has been the last-resort treatment. However, as
SIFK is predominantly localized to a single compartment, and in
view of the favorable outcome of treating unicompartmental
osteoarthritis with UKA, there has been growing interest in treating
SIFK with UKA as an alternative to TKA. The potential benefits
include preservation of the native joint kinematics and proprio-
ception, lower risk of deep vein thrombosis, and less total blood
loss [5]. In a meta-analysis by Jauregui et al. involving 276 UKA
cases performed for SIFK, favorable outcomes were found in the
pain visual analog score, Knee Society Score, and Hospital for
Special Surgery Knee Score at a mean follow-up of 6 years [5]. The
10-year survival rate was 93% with an overall revision rate of 5.5%
[5]. However, the case cohort predominantly described medial
UKA, where only 3 of the 276 included cases involved the lateral
femoral condyle. Although there have been encouraging results for
treating SIFK with UKA, as most cases recruited in the available
literature involve the medial femoral condyle, whether these re-
sults are directly applicable to SIFK of the lateral femoral condyle
remains uncertain. A few studies have compared the outcomes of
TKA and UKA in advanced SIFK but with inconclusive results,
mostly limited by a small cohort size and lack of contemporary
implant designs. A more recent retrospective review by Flury et al.
showed UKA had better functional outcomes than TKA in terms of
the Western Ontario and McMaster Universities Osteoarthritis In-
dex score with a similar complication rate [20]. The size of the
osteonecrotic lesion and surrounding bone edema showed no
correlation to the functional outcome nor implant failure rate in
both groups [20]. However, whether the results were directly
applicable to lateral SIFK cases remain uncertain, as only 3 out of 37
cases in the UKA cohort involved the lateral femoral condyle. Due to
the rarity of the disease entity, there are no available studies of
direct comparisons among the outcomes of surgical modalities for
SIFK of lateral femoral condyle. There are no established clinical or
radiological criteria to guide treatment decision between TKA and
UKA. At the present stage, the choice of TKA vs UKA remains
dependent to surgeons’prior training, individual preference, and
clinical experience.
Summary
In this case report, a middle-aged lady with SIFK of the lateral
femoral condyle was successfully treated with UKA with satisfac-
tory improvement in symptom and functional performance. UKA
appears to be a favorable surgical option, while the optimal treat-
ment modality of SIFK of lateral femoral condyle is still uncertain
due to rarity of the disease entity and paucity of high-quality
evidence.
Conflicts of interest
The authors declare that there are no conflicts of interest.
For full disclosure statements refer to https://doi.org/10.1016/j.
artd.2022.04.002.
Informed patient consent
The author(s) confirm that informed consent has been obtained
from the involved patient(s) or if appropriate from the parent,
guardian, power of attorney of the involved patient(s); and, they
have given approvalfor this information to be published in this case
report (series).
References
[1] Ahlb€
ack S, Bauer GC, Bohne WH. Spontaneous osteonecrosis of the knee.
Arthritis Rheum 1968;11:705e733.
[2] Zywiel MG, McGrath MS, Seyler TM, Marker DR, Bonutti PM, Mont MA.
Osteonecrosis of the knee: a review of three disorders. Orthop Clin North Am
2009;40:193e211.
[3] Yamamoto T, Bullough PG. Spontaneous osteonecrosis of the knee: the
result of subchondral insufficiency fracture. J Bone Joint Surg Am 20 00;82:
858e866.
[4] Karim AR, Cherian JJ, Jauregui JJ, Pierce T, Mont MA. Osteonecrosis of the knee:
review. Ann Transl Med 2015;3:6.
[5] Jauregui JJ, Blum CL, Sardesai N, Bennett C, Henn 3rd RF, Adib F. Uni-
compartmental knee arthroplasty for spontaneous osteonecrosis of the knee:
a meta-analysis. J Orthop Surg (Hong Kong) 2018;26:2309499018770925.
[6] Lotke PA, Ecker ML. Osteonecrosis of the knee. J Bone Joint Surg Am 1988;70:
470e473.
[7] Akamatsu Y, Mitsugi N, Hayashi T, Kobayashi H, Saito T. Low bone mineral
density is associated with the onset of spontaneous osteonecrosis of the knee.
Acta Orthop 2012;83:249e255.
[8] Pareek A, Parkes CW, Bernard C, Camp CL, Saris DBF, Stuart MJ, et al. Spon-
taneous osteonecrosis/subchondral insufficiency fractures of the knee: high
rates of conversion to surgical treatment and arthroplasty. J Bone Joint Surg
Am 2020;102:821e829.
[9] Ochi J, Nozaki T, Nimura A, Yamaguchi T, Kitamura N. Subchondral insuffi-
ciency fracture of the knee: review of current concepts and radiological dif-
ferential diagnoses. Jpn J Radiol 2021;40:443e457.
[10] Sibilska A, G
oralczyk A, Hermanowicz K, Malinowski K. Spontaneous osteo-
necrosis of the knee: what do we know so far? A literature review. Int Orthop
2020;44:1063e1069.
[11] Ohdera T, Miyagi S, Tokunaga M, Yoshimoto E, Matsuda S, Ikari H. Sponta-
neous osteonecrosis of the lateral femoral condyle of the knee: a report of 11
cases. Arch Orthop Trauma Surg 2008;128:825e831.
[12] Gorbachova T, Melenevsky Y, Cohen M, Cerniglia BW. Osteochondral lesions
of the knee: differentiating the most common entities at MRI. Radiographics
2018;38:1478e1495.
[13] Yates PJ, Calder JD, Stranks GJ, Conn KS, Peppercorn D, Thomas NP. Early MRI
diagnosis and non-surgical management of spontaneous osteonecrosis of the
knee. Knee 2007;14:112e116.
C.H. Lo, Y.H.B. Tang / Arthroplasty Today 16 (2022) 15e20 19

[14] MeierC,KraenzlinC,FriederichNF,WischerT,GrizeL,MeierCR,etal.
Effect of ibandronate on spontaneous osteonecrosis of the knee: a ran-
domized, double-blind, placebo-controlled trial. Osteoporos Int 2014;25:
359e366.
[15] Jureus J, Lindstrand A, Geijer M, Roberts D, T€
agil M. Treatment of spontaneous
osteonecrosis of the knee (SPONK) by a bisphosphonate. Acta Orthop
2012;83:511e514.
[16] Miller GK, Maylahn DJ, Drennan DB. The treatment of idiopathic osteonecrosis
of the medial femoral condyle with arthroscopic debridement. Arthroscopy
1986;2:21e29.
[17] Tanaka Y, Mima H, Yonetani Y, Shiozaki Y, Nakamura N, Horibe S. Histological
evaluation of spontaneous osteonecrosis of the medial femoral condyle and
short-term clinical results of osteochondral autografting: a case series. Knee
2009;16:130e135.
[18] Tírico LEP, Early SA, McCauley JC, Bugbee WD. Fresh osteochondral allograft
transplantation for spontaneous osteonecrosis of the knee: a case series.
Orthop J Sports Med 2017;5:2325967117730540.
[19] Akgun I, Kesmezacar H, Ogut T, Kebudi A, Kanberoglu K. Arthroscopic
microfracture treatment for osteonecrosis of the knee. Arthroscopy 2005;21:
834e843.
[20] Flury A, Weigelt L, CamenzindRS, Fritz B, Hasler J, Baumgaertner B, et al. Totaland
unicondylar knee arthroplasty are equivalent treatment options in end-stage
spontaneous osteonecrosis of the knee, and the size of the lesion has no influ-
ence on the results. Knee Surg SportsTraumatol Arthrosc 2021;29:3254e3261.
C.H. Lo, Y.H.B. Tang / Arthroplasty Today 16 (2022) 15e2020