Surgical Science, 2011, 2, 232-241
doi:10.4236/ss.2011.25052 Published Online July 2011 (http://www.SciRP.org/journal/ss)
The Evaluation of Refrigerated and Frozen Osteochondral
Allografts in the Knee
Albert Washington Pearsall IV1, Sudhakar Govindarajulo Madanagopal1, Joseph Allan Tucker2
1Department of Orthopaedic Surgery, College of Medicine, University of South Alabama, Mobile, US
2Departmen t o f Pathology, College of Medicine, University of South Alabama, Mobile, US
Received May 6, 2011; revised June 27, 2011; accepted July 10, 2011
Abstract
Between 1998 and 2002, 25 patients who were treated with a refrigerated or frozen allograft were evaluated.
The mean patient age was 48 years. The mean lesion size was 4.5 cm2. Validated outcome instruments [Knee
Society Score, Western Ontario and McMaster University Score] were used. Clinical and radiographic eval-
uations were performed pre-operatively and at the most recent follow-up. Histological and electron micro-
scopic analysis was performed on grafts prior to implantation. Clinical follow-up averaged 46 months (range
24 - 60 months). The Western Ontario and McMaster University Score improved from 46 + 24 to 66 + 22 (p
= 0.003). The Knee Society Score improved from 104 + 43 to 132 + 42 (p = 0.01). No correlation was noted
between graft type and histological or electron microscopy scoring. Post-operative mechanical alignment
was not correlated with an improvement in Western Ontario and McMaster University Score (p = 0.19) or
Knee Society Score (0.27). Six patients (24%), all refrigerated allografts, were failures and underwent knee ar-
throplasty. Seventy-six percent of implanted frozen and refrigerated osteochondral allografts are in place 4
years after surgery. Frozen allografts appear to be surviving as well as refrigerated grafts. The use of mag-
netic resonance imaging may enable the evaluation of graft incorporation and articular cartilage integrity.
Keywords: Allograft, Refrigerated, Frozen, Knee, Transplantation
1. Introduction
Biologic treatment options for large full thickness osteo-
chondral lesions include microfracture, autologous carti-
lage transplantation, mosaicplasty and refrigerated or
frozen osteochondral allograft transplantation [1-6]. The
result of these treatments is partial defect filling with
fibrocartilage consisting of predominately type I collagen.
Fibrocartilage has diminished resilience and a predilec-
tion for deterioration over time [2,3,5,7-10].
The use of fresh osteochondral allografts in the treat-
ment of full thickness articular cartilage defects has been
well documented, with success rates of 75% reported at 5
years, slightly deteriorating to 63% at 14 years [8,11-18].
The term “fresh” usually indicates graft harvest within
24 hours of the donor’s death and a time from graft har-
vest to implantation of 7 days or less [2,9,10,12,13,
19-24]. Deep frozen allografts have also been used for
reconstruction of osteoarticular defects. However, au-
thors have cited the diminished cell viability and poten-
tial matrix degeneration that can occur after freezing
hyaline cartilage [11,25-28].
Previously, authors have published the viability results
of stored refrigerated allografts [3,29,30]. However, the
authors are unaware of data correlating patients’ func-
tional outcome with radiographic evaluation and histo-
logical/ electron microscopy grading of refrigerated and
frozen allografts at the time of implantation. The purpose
of the current study was to clinically and radiographi-
cally evaluate patients who underwent refrigerated or
frozen allograft transplantation. In addition, histologi-
cal/electron microscopy grading of the allograft and ra-
diographic evaluation of the affected knee was analyzed
in relationship to functional outcome at an average of 46
months follow-up.
2. Materials and Methods
2.1. Patient Data
The current study was approved by the Institutional Re-
view Board at our institution prior to implementation. All

A. W. PEARSALL IV ET AL. 233
study patients gave informed consent prior to being en-
rolled in the study. Between 1998 and 2002, 26 patients
underwent osteoarticular transplantation of the femur
and/or patella with a refrigerated or frozen allograft from
which histologic and electron microscopic data was
available at the time of implantation. Inclusion criteria
were as follows: 1) Tegner 3 or greater activity level; and
2) a contained articular cartilage defect amenable to a
non-structural osteoarticular graft; 3) articular cartilage
damage limited to 1 or 2 compartments; 4) biomechani-
cal knee alignment that was less than 5° of varus or val-
gus or correctable with a distal femoral or proximal tibial
osteotomy; and 5) failure of conservative measures in-
cluding non-steroidal anti-inflammatory medications and
physical therapy for a minimum of 3 months. Body mass
index greater than 30 and chronological age were not
used as exclusionary criteria from surgery. Patients
whose lifestyle included less demanding activities were
encouraged to undergo a unicondylar or total knee ar-
throplasty.
Prior to surgery, all patients underwent clinical evalu-
ation, standing radiographic imaging of both knees, and
magnetic resonance imaging of the affected knee. Spe-
cific magnetic resonance imaging sequences (fast spin
echo) were performed to assess the articular cartilage of
the patella, femur and tibia. All images were reviewed by
the senior author and a board certified, fellowship-
trained radiologist.
2.2. Operative Technique
All allograft transplants were performed through a
mini-arthrotomy unless a tibial tubercle osteotomy was
performed for a patellar and/or trochlear defect. After the
recipient defect was measured, the bone was reamed to a
depth of 8 and 12 mm. The donor allograft was cored
with a coring device (Arthrex, Inc., Naples, Florida) with
a minimum diameter of 18 mm. The graft was press-fit
into the recipient site with the articular surfaces congru-
ent. All grafts were placed with less than 1 mm of do-
nor-recipient articular surface incongruity. Early in the
study, 1 graft (patella) was secured with an intra-articular
screw. No subsequent grafts underwent fixation.
All tibial tubercle osteotomies were performed ac-
cording to the technique described by Fulkerson and se-
cured with two 3.5 mm or 4.5 mm screws [24] .All high
tibial osteotomies were performed with a lateral closing
or medial opening wedge. All distal femoral osteotomies
were performed with a lateral opening wedge. For all
opening wedge osteotomies, frozen allograft corticocan-
cellous bone graft was used to fill the osteotomy defect.
2.3. Postoperative Treatment
All patients undergoing an isolated mini-arthrotomy
were discharged the following day. Patients undergoing a
tibial tubercle osteotomy, high tibial osteotomy or distal
femoral osteotomy remained in the hospital 48 to 72
hours. For all inpatients, continuous passive motion was
instituted on the first post-operative day. Continuous
passive motion was continued on an outpatient basis 3 to
6 hours per day for 3 weeks post-operatively. Patients
undergoing isolated allograft transplantation were kept
toe-touch weight bearing for 4 weeks and then pro-
gressed to full weight bearing over 2 weeks. All patients
undergoing a tibial tubercle osteotomy, high tibial os-
teotomy or distal femoral osteotomy were kept non-weight
bearing for 6 weeks and progressed to full weight- bear-
ing over the next 2 weeks. Heavy labor and athletic ac-
tivity were delayed until 6 months after surgery.
2.4. Clinical and Radiographic Assessment
All patients underwent radiographic evaluation preopera-
tively and at yearly intervals. Two standardized radio-
graphic views (standing antero-posterior and skyline)
were obtained by a trained radiological technician. In
addition, standing long-leg antero-posterior radiographs
were obtained pre-operatively and at the most recent
follow-up. A trained research assistant blinded to the
patient’s clinical data measured each film. Each film was
assessed for the area of greatest joint space narrowing
(Grade 3 = > 3mm joint space, Grade 2 = < 3mm of joint
space but not bone on bone, Grade 1= bone on bone) in
specific areas (Figure 1). Based upon these measure-
ments, the patient was given a composite score ranging
from 3 to 6 which was used for later analysis. Long leg
weight-bearing radiographs were measured by one of the
authors. The biomechanical axis was determined by the
intersection of a line drawn from the center of the femo-
ral head to the center of the tibial plateau and a line
drawn from the center of the talus to the center of the
tibial plateau.
All patients underwent clinical evaluation including
knee range of motion. Patients completed the Knee Soci-
ety Score, Western Ontario and McMaster University
(WOMAC) Score preoperatively and at yearly intervals
after surgery. Failure was defined as conversion of a
transplanted graft to a unicondylar or total knee arthro-
plasty. All failures were included in the overall analysis
and also analyzed separately.
2.5. Refrigerated Allografts
All grafts were aseptically processed within 72 hours of

A. W. PEARSALL IV ET AL. 234
Figure 1. Knee radiographic grading form. A grade from
one to three is given for the medial and lateral joint spaces
within the knee and patellofemoral joint.
the donor’s death and procedures were performed in ac-
cordance with the guidelines established by the Ameri-
can Association of Tissue Banks. Donors’ blood was
screened for malignancies, autoimmune and certain neu-
rological disorders, and any high-risk behavior. Donor
blood and tissue was screened for human immunodefi-
ciency virus-1 and human immunodeficiency virus-2
antibodies, Hepatitis B and C antibodies, human T-lym-
photropic Virus I and II, syphilis, and polymerase chain
reaction testing for human immunodeficiency virus-1.
Harvested osteochondral grafts were aseptically clea-
nsed with saline pulse lavage in order to remove blood
and fat from the cancellous bone. All grafts were pre-
-soaked in antibiotic storage medium. Grafts were trans-
ferred to double sterile pouches containing storage media
to maximize chondrocyte viability during storage (IN-
CELL Corp., San Antonio, Texas). Only grafts demon-
strating no growth after 7 day microbial culturing were
released for distribution. Grafts were refrigerated and
stored at 2° - 8° Celsius with a shelf life of 6 weeks from
the date of processing (Regeneration Technologies Inc.,
Alachua, Florida).
2.6. Frozen Allografts
All grafts were aseptically processed within 72 hours of
the donor’s death and procedures were performed in ac-
cordance with the guidelines established by the Ameri-
can Association of Tissue Banks. Donors’ blood was
screened for malignancies, autoimmune and certain neu-
rological disorders, and any high-risk behavior. Donor
blood and tissue was screened for human immunodefi-
ciency virus-1 and human immunodeficiency virus-2
antibodies, Hepatitis B and C antibodies, human T-
lymphotropic Virus I and II, syphilis, and polymerase
chain reaction testing for human immunodeficiency vi-
rus-1. All grafts were stored at –70° Fahrenheit until the
day of use.
2.7. Cartilage Preparation and Histologic
Scoring
Immediately upon opening the allograft in the operating
room, three 5 mm plugs were sterilely harvested from a
peripheral area of articular cartilage with an Osteo-
chondral Autograft Transfer System (OATS) harvester
(Arthrex, Naples, Florida). The sample site was free from
any visual cartilage defects. All plugs were placed in a
sterile container with a small amount of normal saline
and sealed for transport to the pathology and flow cy-
tometry laboratories for evaluation. The time from plug
harvest to processing was approximately 30 minutes for
all specimens.
For histology a portion of fresh cartilage was cut from
the bone surface, fixed in 10% buffered formalin, proc-
essed to paraffin, sectioned at 5 um, and stained with
hematoxylin and eosin. Three slides were prepared from
the plugs and sent for evaluation. The slides were scored
blindly and the scores averaged. The average score was
used for data analysis. The following scoring system was
utilized for each slide: 0: all cells appeared lethally in-
jured; 1+: majority of cells exhibited marked to lethal
injury; 2+: minority of cells exhibited marked to lethal
injury; 3+: all cells appear viable. Severe pyknosis and
cell lysis were judged to represent marked to lethal in-
jury.
For electron microscopy, fresh tissue was placed in
3% glutaraldehyde (0.1 N sodium cacodylate buffer, pH
7.2), post-fixed in 1% osmium tetroxide, and embedded
in epoxy resin. Sections (1 um) were stained with tolu-
idine blue. Thin sections were stained with uranyl acetate
and lead citrate and examined in a Phillips CM100 elec-
tron microscope (FEI Company, Hillsboro, Oregon). The
scoring scheme above was also applied to the ultrastruc-
tural findings. Features such as severe pyknosis, ob-
scured detail of the organelles, and rupture of the plasma
membrane were considered indicators of lethal injury,
and lesser degrees of cytoplasmic contraction, cytoplas-
mic blebs, and accumulation of myelin figures were con-
sidered indications of marked injury (Figures 2(a)-2(c)).

A. W. PEARSALL IV ET AL. 235
(a)
(b)
(c)
Figure 2. (a)-(c) Pre-operative and post-operative radio-
graphs and intraoperative radiographs and intra-operative
images of an active male farmer who underwent a distal
femoral opening wedge osteotomy and lateral femoral condyle
refrigerated osteoarticular allograft. (a) Pre-operative stand-
ing antero-posterior radiograph. The patient’s preoperative
KSS score was 175. (b) Intra-operative image from the pre-
vious patient showing two refrigerated osteoarticular plugs
in place in the lateral femoral condyle. (c) Post-operative
standing antero-posterior radiograph of the same patient.
His p ost - oper a tive K S S sc ore at the l a test fo l l o w- u p was 190.
2.8. Statistical Analysis
Statistical analyses were performed using appropriate
procedures of the JMP software system (SAS Inc., Carey,
NC). Means for demographics, graft size, follow-up,
shelf time (time from harvest to implantation), harvest
time (time of death to time of harvest), electron micros-
copy score and histology score were calculated. Correla-
tion analysis were performed to determine if a relation-
ship existed between electron microscopy score, histol-
ogy score, outcome score and any other variable. Sig-
nificance was determined at the 0.05 level.
3. Results
Twenty-five patients (96%) with complete data were
available for follow-up. The average age for the overall
group was 48 years. The average follow-up for the group
was 46 months. The average body mass index of the
group was 32. The average graft size for the group was
4.5 cm2. Pre-operative knee range of motion was less in
the frozen group when compared to the refrigerated
group (p = 0.01) (Table 1).
Both groups improved after surgery. Post-operative
WOMAC Score, Knee Society Score and knee range of
motion values were 66, 132, and 116 respectively (Table
2) (Figures 2(a)-2(c)). Post-operative improvement in
WOMAC Scores and Knee Society Scores, and knee
range of motion was analyzed in relation to graft type.
Greater improvement in WOMAC and Knee Society
Scores was noted in the frozen group when compared to
the refrigerated group (p = 0.07) (Figures 3(a)-3(b)). An
improvement in knee range of motion was noted in fro-
zen patients compared refrigerated allograft patients (p =
0.02) (Table 2).
Table 1. Demographic variables for refrigerated and frozen
allograft patients.
Variable Refrigerated Frozen p-value
Age 44 (17-69) 57 (35-66) 0.02*
Sex (male/fem ale) 10/8 1/8 0.05*
BMI 30 (20-48) 35 (25-43) 0.11
Follow-up (month s)47(24-60) 44(35-57) 0.47
Number of plugs 2.6 (1-5) 2.2 (1-4) 0.53
Size (cm2) 4.2 (2.5-7.3) 5.3 (2.0-8.5) 0.20
Pre-op pain 3/4 (1-4) 4/4 (1-4) 0.15
Pre-op WOMAC 52 (6-94) 35 (19-63) 0.1
Pre-op KSS 113 (30-192) 85 (30-130) 0.12
Pre-op ROM 118 (95-135) 99 (50-130) 0.01*
*Statistically significant.

A. W. PEARSALL IV ET AL. 236
Table 2. Overall pre and post-operative outcome scores and
outcome improvement in refrigerated and frozen allograft
patients.
Outcome
Measure Pre-operative score Post-operative score P value
WOMAC 46 +/– 24 66 +/– 22 0.003 *
KSS 104 +/– 43 132 +/– 42 0.01*
Range of motion 112 +/– 19 116 +/– 11 0.22
Outcome
Measure RefrigeratedFrozen P value
WOMAC
improvement 10 +/– 21 28 +/– 21 0.07
KSS
improvement 14 +/– 35 49 +/–52 0.07
ROM
improvement –3 +/– 15 20 +/– 30 0.02*
*Statistically significant.
Radiographic measurements were made on standing
antero-posterior (AP) and skyline views. Medial and
lateral tibio-femoral joint spaces were given a score from
1 to 3, while medial and lateral patello-femoral joint
spaces were given a score from 1 to 3. The baseline
group AP radiographic score was 5.1 which increased at
follow-up to 5.3 (p = 0.06). The baseline patella score for
the group was 5.6 which increased at follow-up to 5.7
(Table 3).
Mechanical axis was measured in all patients without
a tibial tubercle osteotomy. The average pre-operative
and post-operative mechanical axes were 174° and 177°
respectively (p = 0.08). When post-operative alignment
was analyzed in relation to outcome improvement, no
correlation was noted (Table 4).
Due to the small numbers, the 4 histological and elec-
tron microscopy scores were combined (scores zero and
one = Grade 1/scores two and three = Grade 2). When
pre-operative WOMAC and Knee Society Scores were
compared, there was no difference between groups. No
correlation was noted between graft type and histological
or electron microscopy scoring. Therefore, no difference
in chondrocyte viability was noted on histological or
electron microscopy between refrigerated and frozen
allografts at the time of implantation. No correlation was
noted between the histological or electron microscopy
grading systems and any outcome tool (Table 5).
Improvement in outcome was analyzed in relation to
the number of sites undergoing grafting. No difference in
outcome improvement was noted between single or mul-
tiple site grafts (Knee Society Score p = 0.7; WOMAC
Score p = 0.5; range of motion p = 0.9).
No difference in post-operative scores was noted be-
tween the 2 groups (p = 0.8, p = 0.7). However, poorer
(a)
(b)
Figure 3. (a), (b) Pre-operative and post-operative radio-
graphs of a homemaker with 22 degrees of combined femo-
ral and tibial varus. She underwent combined distal femo-
ral closing wedge and medial tibial opening osteotomies. A
frozen osteoarticular allograft was used to reconstruct the
medial femoral condyle. (a). Pre-operative stan- ding an-
tero-posterior radiograph. The patient’s pre-ope- rative
KSS score was 59. (b). Post-operative standing an-
tero-posterior radiograph of the same patient. Her
post-ope- rative KSS score at the latest follow-up was 185.

A. W. PEARSALL IV ET AL. 237
Table 3. Pre-operative and post-operative radiological
scores for all patients (n = 25).
Score Pre-operative Post-operative P value
AP Score 5.1 +/– 0.96 5.3 +/– 1.18 0.6
Patella Score 5.6 +/– 1.12 5.7 +/– 1.36 0.5
Table 4. Analysis of post-operative mechanical alignment in
relation to improvement in outcome score.
Outcome Varus
(n = 12) Neutral (n = 2) Valgus (n = 3) P value
WOMAC 6 +/– 6 34 +/– 15 24 +/– 130.19
KSS 31 +/– 12 54 +/– 31 –9 +/– 250.27
Knee Range
of motion 6.4 +/– 6.7 22.5 +/– 17 1.6 +/– 140.33
Table 5. Evaluation of histological and electron microscopy
grades compared to outcome measures.
Histology grading versus outcome measures
Outcome measures Gr a de 1 Grade 2 P value
WOMAC improvement 10.66 21.63 0.25
KSS improvement 24.41 24.81 0.9
ROM improvement –1.6 10.45 0.2
Electron microscopy grading versus outcome measures
Outcome measures Grade 1 Grade 2 P value
WOMAC improvement 13.73 21.75 0.5
KSS improvement 23.93 41.25 0.5
ROM improvement 1.4 15.25 0.29
pre-operative knee range of motion, Knee Society and
WOMAC scores in patients receiving a frozen allograft
(p = 0.01, p = 0.1, p = 0.1) resulted in a greater net
change in outcome in the frozen allograft group.
Six patients (24%) who underwent a knee arthroplasty
were considered failures and analyzed separately. All
failures were refrigerated allografts. We evaluated histo-
logical and electron microscopy findings at the time of
implantation with success or failure of the graft. No fail-
ures were noted if the histology score was Grade 2 (p =
0.02) or if the electron microscopy score was Grade 2 (p
= 0.4).
4. Discussion
The use of fresh allografts for the treatment of full
thickness articular defects is well documented [2,15,
31-34]. Fresh osteochondral shell allografts (<1 cm of
subchondral bone) provide the greatest likelihood of
chondrocyte survivability, while reducing immunogenic-
ity by decreasing the exposure of white cells found in
cancellous bone [1]. Various authors have indicated that
the long-term survival of an allograft is dependent upon
viable chondrocytes with a high level of donor deoxyri-
bonucleic acid (DNA) to replenish the transplanted ma-
trix [35]. Currently, fresh allografts provide the largest
population of viable donor chondrocytes for transplanta-
tion.
Although fresh osteochondral allografts provide an
excellent source of live chondrocytes, their use is not
without drawbacks. Foremost is the need for the graft to
be as fresh as possible to maintain chondrocyte viability,
yet not expose the recipient to infection at the time of
implantation. Currently, all published reports of osteo-
chondral grafts using the term “fresh” have cited a time
from graft harvest to implantation of 7 days or less
[4,36-44]. In the current study, the time from graft har-
vest to implantation was greater than 7 days for all
non-frozen grafts. Noting this, we chose to label these
grafts as “refrigerated”, rather than “fresh”.
Recent data have indicated good results with the use of
refrigerated osteochondral allografts. Emmerson et al.
reported 72% good/excellent results at 7.7 years with
refrigerated allografts averaging 7.5 cm in size implanted
for osteochondritis dissecans of the femoral condyle.
[45]. McCulloch et al. reviewed 25 consecutive patients
who underwent refrigerated osteochondral allograft
transplantation for defects in the femoral condyle. Over-
all, patients reported 84% satisfaction with their results
[46]. Williams et al. reported on 19 patients who were
treated with “fresh stored” allografts maintained for an
average of 30 days prior to implantation. Mean lesion
size was 602 mm2. At the most recent follow-up, the
mean Short Form-36 score improved to 66 (+/–24) and
the mean score on the Activities of Daily Living Scale
increased to 70 (+/–22) [47].
The outcome tools used in the current study were not
identical to those used by other authors [46,47]. However,
both the WOMAC and Knee Society Scores are vali-
dated outcome instruments reported in the literature for
the evaluation of patients with arthritic conditions [48-
50]. Indeed, previous studies of function in osteoarthritis
have shown that WOMAC Score is more sensitive to
change and has greater efficiency than other instruments
used to assess osteoarthritis, including the SF-36 Health
Survey [51,52].
In the current study we noted clinical improvement in
patients implanted with a refrigerated or frozen allograft.
Many of these patients had malalignment or patel-
lofemoral disease requiring a concomitant osteotomy.
Whe- ther the osteotomies themselves played a larger

A. W. PEARSALL IV ET AL. 238
role then the transplantation in improved patient outcome
is unclear.
Numerous animal studies have indicated that har-
vested osteochondral allografts can be safely preserved
up to 28 days with overall maintenance of biomechanical
properties, cell matrix collagen content, and permeability
[53-56]. Wayne et al. demonstrated that osteochondral
shell grafts could be stored in culture medium at 4° Cel-
sius for up to 60 days without deterioration of collagen
content, proteoglycan amount, or histologic appearance
[56]. Pearsall et al. reported on 16 refrigerated osteo-
chondral allografts that underwent histologic and ultra-
structural examination prior to implantation. The authors
found an inverse correlation between matrix staining and
time to implantation (p < 0.05) and reported an average
time from harvest to implantation of 30 days (range: 17
to 44 days) [57]. Despite the aforementioned animal and
histologic/electron microscopy data, the authors are un-
aware of published data correlating pre-implantation
histologic and electron microscopy findings with clinical
outcome.
The use of deep frozen allografts for the treatment of
osteoarticular defects has also been reported, with cited
failure rates as high as 25% [2,15,58]. Reasons cited for
failure include slow incorporation, diminished chondro-
cyte survival, and subsequent matrix degeneration [1,58].
Pritzker and others noted that freezing cartilage kills
chondrocytes [4,23,59-61]. Despite reports of poor
chondrocyte viability at the time of frozen allograft im-
plantation, we detected no significant difference in out-
come between our refrigerated and frozen allograft pa-
tients. Moreover, when pre-operative scores were exam-
ined, the frozen allograft group had significantly worse
scores at the time of implantation, indicating that these
patients were actually doing better at the latest follow-up.
There are several weaknesses to the current study. The
analysis included a limited number of patients. Therefore,
statistical significance could not be determined for sev-
eral variables. There was no follow-up magnetic reso-
nance imaging of the study patients to assess incorpora-
tion of the implanted graft. Previous authors have hy-
pothesized that early magnetic resonance imaging ab-
normalities represent non-specific post-operative change,
whereas persistent abnormalities represent immune-related
injury [62]. Further follow-up of the current patient co-
hort will include magnetic resonance imaging to evaluate
graft incorporation and analyze the findings in relation to
clinical outcome. The histological and electron micros-
copy evaluation was performed by a single pathologist.
However, the evaluator has over 20 years of expertise in
this area and interpreted all specimens blinded. Although
several of the electron microscopy and histologic scores
were low, these findings may be attributable to the need
to refine the technique of electron microscopy and his-
tologic assessment in articular cartilage. Finally, al-
though the current study assessed chondrocyte viability,
it did not evaluate chondrocyte function. Various authors
have reported on the mechanical properties of articular
cartilage, including an assessment of cartilage stiffness
[63-65].
In conclusion, 76% of implanted frozen and refriger-
ated osteochondral allografts in the current study are still
in place at 4 years after surgery. At the latest follow-up,
frozen allografts are surviving as well as refrigerated
grafts. The reason for this finding is unclear, as the ma-
jority of these grafts were used in salvage cases in older
patients. Long term follow-up is needed to assess clinical
outcome, while the use of magnetic resonance imaging
may be beneficial to evaluate graft incorporation and
articular cartilage integrity.
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