Iatrogenic Patella Baja Following Primary Total Knee Arthroplasty: Is the Patellar Tendon to Blame?

Abstract

Patellar tendon shortening (PTS) following primary total knee arthroplasty (TKA) is thought to occur because of excessive soft tissue tensioning during wound closure. Few studies have examined the incidence of acute PTS in TKA patients. The purpose of this prospective study was to evaluate the incidence and clinical implications of acute PTS after primary TKA. All patients undergoing primary TKA for osteoarthritis (OA) from January 2024 through April 2024 by a single, fellowship-trained surgeon were included. Patient demographics and range of motion (ROM) were recorded preoperatively. Range of motion and physical therapy (PT) requirements were recorded at 6-week follow-up. Patellar tendon length was determined by the Insall-Salvati ratio (ISR) and measured preoperatively, on postoperative day (POD) 0, and at 6 weeks following surgery. Significant PTS was defined as a decrease in the ISR of ≥10%. In total, 89 patients were included in the analysis. Of these, 54 (60.7%) patients experienced significant PTS and 35 (39.3%) did not experience significant PTS immediately following TKA. Preoperative ISR and ROM was similar between cohorts; however, on POD 0, the ISR decreased by 21.9 ± 8.7% in the significant PTS cohort versus 0.8 ± 10.9% (p < 0.001) in the insignificant PTS cohort. From POD 0 to 6 weeks postoperatively, ISR increased by 25.0 ± 15.8% in the significant PTS cohort versus 7.6 ± 12.0% in the insignificant PTS cohort (p < 0.001). The ISR decreased by 2.9 ± 10.9% for patients in the significant PTS cohort and increased by 5.7 ± 7.9% for patients in the insignificant PTS cohort (p < 0.001). There was no significant difference in PT requirements or ROM between cohorts at 6-week follow-up. Patellar tendon shortening following TKA resolved by 6 weeks postoperatively; no ROM deficits or additional PT requirements were found to exist between cohorts.

Keywords

patellar tendon shortening - postoperative outcomes

Introduction

Primary total knee arthroplasty (TKA) is an extremely successful procedure for the resolution of pain and functional impairment from end-stage knee osteoarthritis (OA). However, complications involving the patellofemoral joint following TKA remain common indications for revision TKA.[1] Of the described complications, patella baja is characterized by (1) distal positioning of the patella in the femoral trochlea, (2) patellar tendon shortening (PTS), and (3) decreased distance between the inferior pole of the patella and articular surface of the tibia.[2] The incidence of significant PTS, defined as a decrease of 10% or greater in Insall-Salvati ratio (ISR) after TKA, has been reported to range from 34 to 38% at 1 and 5 years postoperatively.[3] [4] Clinically, patients with PTS following TKA may present with anterior knee pain, crepitus, reduced range of motion (ROM), and reduced functional outcome scores.[5] [6] The exact etiology, however, has yet to be completely understood and is likely multifactorial.[3] [7] Ischemic injury sustained intraoperatively via lateral retinacular release, tourniquet use, and infrapatellar fat pad excision has been postulated to cause PTS due to peripatellar fat pad contractures or quadriceps insufficiency.[3] [7] [8] [9] [10] Alternatively proposed etiologies of PTS include the formation of new bone, tethering of the tendon by intra-articular fibrous bands, and scarring of the patella to the retinaculum.[11] [12]

In addition to the previously proposed etiologies of PTS that result in chronic shortening, acute PTS may occur in the immediate postoperative period as a result of aberrant soft tissue tensioning during surgical wound closure. Arthrotomy closure in flexion has been shown to contribute to early ROM recovery and reduced physical therapy (PT) requirements,[13] [14] although Masri et al found no significant difference between capsular closure in flexion versus extension.[15] Recently, Clark et al found closing either the arthrotomy and skin or skin only in extension led to reduced postoperative patellar height when compared to in flexion; however, patellar height reduction was not sustained at 1 year postoperatively.[16]

These findings suggest some restoration of patellar length postoperatively but fail to provide specific details pertinent to when the greatest increase in length may occur and over what time period.[16] Additionally, it is unclear whether any limitations in function were sustained due to this finding or if any intervention was required to return to the patient's baseline functional status. Understanding when the patellar tendon achieves the greatest restoration in length in patients with PTS after TKA can help guide surgeons in determining when conservative or more invasive interventions may be necessary. Although techniques have been described in the literature for the management of patella baja after TKA, there is currently no gold-standard procedure or algorithmic approach for its management.[17] The purpose of this study was to identify when postoperative patellar tendon lengthening occurs in patients with iatrogenic patella baja secondary to significant PTS following TKA and during what early, postoperative time period patients achieve the greatest restoration in patellar tendon length. We hypothesize that patients will experience significant PTS in the acute postoperative period following TKA but will subsequently achieve patellar tendon lengthening by 6 weeks postoperatively.

Methods

Study Population

This study was evaluated by our institutional review board (IRB) and determined to be exempt from review. A consecutive series of patients ≥18 years of age undergoing unilateral, primary TKA for end-stage knee OA from January through April 2024 by a single, high-volume, fellowship-trained arthroplasty surgeon were included in our study. Patients were excluded if they (1) had a preoperative diagnoses of connective tissue diseases, disorders associated with joint hypermobility, or diseases with pro-fibrotic processes, (2) a history of ipsilateral knee trauma, surgery, or infection, or (3) had preoperatively diagnosed patella baja, defined as an ISR <0.8 or patella alta, defined as an ISR >1.2.

Data Collection

Data on patient demographics including age, sex, race, ethnicity, body mass index (BMI), American Society of Anesthesiologist (ASA) score, age-adjusted Charlson Comorbidity Index (CCI) score,[18] and preoperative patient-reported outcomes measures (PROMs) including the knee injury and osteoarthritis outcome score for joint replacement (KOOS-JR) and Veteran's Rand 12 Item Health Survey (VR-12) were prospectively recorded. Active knee flexion, extension, and total ROM, defined as extension subtracted from flexion, was measured by the operating surgeon using a goniometer and recorded preoperatively at time of surgery sign-up (within 1 month of TKA) and at 6-week follow-up appointments. Perioperative data including tourniquet use and time, estimated blood loss (EBL), and tranexamic acid (TXA) use, in addition to postoperative data including PT requirements and manipulation under anesthesia (MUA), were also prospectively recorded.

Radiographic Measurements

Patellar tendon length was measured via ISR on preoperative, POD 0, and 6-week postoperative radiographs as per the methods described by Insall and Salvati and validated by Cabral et al.[19] [20] The ISR is measured by drawing one line from the proximal tibial tubercle to the inferior pole of the patella and dividing that distance by that measured by drawing another line from the superior to inferior pole of the patella. Significant PTS was defined as a decrease in the ISR by ≥10%, consistent with prior literature.[3] [7] Measurements were performed using our institution's Picture Archiving and Communication System (PACS) (Sectra IDS7 v.24.2).

Study Outcomes

The primary outcome of the study was the incidence of significant PTS following primary TKA and changes in patellar tendon length from preoperative radiographs to those taken on POD 0 and at 6 weeks postoperatively. Secondary outcomes included ROM, PT attendance, and MUA for postoperative stiffness.

Surgical Technique

All primary TKAs were performed by one, fellowship-trained, high-volume arthroplasty surgeon at our institution. Spinal anesthesia was utilized in all instances unless contraindicated. After sterile preparation and draping of the patient, the operative leg was exsanguinated prior to padded, pneumatic tourniquet inflation. A medial parapatellar approach was utilized to gain access to the knee joint. All cuts were checked using cutting and alignment guides prior to trialing components. Once the knee was felt to be well aligned and stable throughout range of motion with trial components, bony surfaces were irrigated prior to cementation of final components using pressurized vacuum-mixed cement. All patients received P.F.C. Sigma Knee System (DePuy Synthes, Warsaw, IN) components, and specifically, posterior stabilized femoral components. Additionally, all patellae were resurfaced. The knee was held in 30 degrees of flexion while the wound was closed in layers using #1 Vicryl for arthrotomy closure, 2-0 Monocryl for subcuticular closure, and Dermabond or staples for skin closure, with a soft, compressive dressing applied.

Postoperative Protocol

All patients were instructed to weight-bear as tolerated and met once with a physical therapist prior to discharge. Patients were instructed to follow a detailed, home stretching protocol with emphasis placed on active knee flexion to 90 degrees in the first 2 weeks following surgery. Patients who did not achieve 90 degrees of active knee flexion by 2 weeks postoperatively were prescribed formal, outpatient PT.

Statistical Analyses

Descriptive statistics were calculated first to understand the overall distribution of the data. Independent t-tests or analysis of variance was used to compare continuous variables for parametric data and Mann-Whitney U or Kruskal-Wallis tests for nonparametric data. Chi-square or Fisher's exact tests were used to compare categorical data. Data are either presented as mean (standard deviation) or count (%). Significance was determined at P-value <0.05. All statistical analyses were done using R Studio (Version 4.1.2, Vienna, Austria).

Final Cohort

A total of 89 patients were included in the final analysis. The mean age was 66.9 ± 8.3 years, and 61 (68.5%) patients were women. In our final cohort, 62 (69.7%) patients identified as White and 24 (27.0%) identified as Black, while 87 (97.8%) patients identified as not Hispanic or Latino and 2 (2.3%) identified as either Hispanic or Latino. Mean ASA score was 2.5 ± 0.6, BMI was 33.0 ± 5.6 kg/m², and age-adjusted CCI score was 3.9 ± 1.9. Preoperatively, the mean KOOS-JR score for our cohort was 42.7 ± 20.8 and VR-12 score was 30.5 ± 15.5 ([Table 1]). Preoperative ISR was 1.1 ± 0.2 while active ROM was 101 ± 19.6 degrees, specifically 4.4 ± 4.5 degrees of extension and 107 ± 13.4 degrees of flexion ([Table 2]).

Table 1 Demographics of all patients who underwent primary total knee arthroplasty (n = 89)

	All patients
	(n = 89)
Age (years)	66.9 ± 8.3
Sex
Man	28 (31.5)
Woman	61 (68.5)
Race
Black	24 (27.0)
Other	3 (3.4)
White	62 (69.7)
Ethnicity
Hispanic or Latino	2 (2.3)
Not Hispanic or Latino	87 (97.8)
Laterality
Left	39 (43.8)
Right	50 (56.2)
ASA	2.5 ± 0.6
BMI	33.0 ± 5.6
CCI	3.9 ± 1.9
Preoperative KOOS-JR	42.7 ± 20.8
Preoperative VR-12	30.5 ± 15.5

Abbreviations: ASA, American Society of Anesthesiologist's physical status classification score; BMI, body mass index; CCI, age-adjusted Charlson comorbidity index; KOOS-JR, Knee Injury and Osteoarthritis Outcome Score for Joint Replacement; VR-12, Veterans Rand 12 Item Health Survey.

Note: Values given as mean ± SD or N (%).

Table 2 Clinical characteristics of all patients who underwent primary total knee arthroplasty (n = 89)

	All patients
	(n = 89)
Anesthesia
General	5 (5.6)
Spinal	84 (94.4)
Cut to close (min)	84.3 ± 12.6
EBL (mL)	5.3 ± 9.5
Tourniquet use
Yes	89 (100)
Time (min)	84.4 ± 14.9
TXA use
No	37 (41.6)
Yes	52 (58.4)
Dose (mg)	981 ± 169
Significant postoperative PTS
No	35 (39.3)
Yes	54 (60.7)
ISR
Preoperatively	1.1 ± 0.2
POD 0	1.0 ± 0.2
6 weeks PO	1.1 ± 0.2
∆ ISR (%)
Preoperatively to POD 0	−13.6 ± 14.1
POD 0 to 6 weeks PO	18.0 ± 16.7
Preoperatively to 6 weeks PO	0.5 ± 10.6
Flexion (degrees)
Preoperatively	107 ± 13.4
6 weeks PO	110 ± 19.0
Extension (degrees)
Preoperatively	4.4 ± 4.5
6 weeks PO	1.9 ± 4.1
Total ROM (degrees)
Preoperatively	101 ± 19.6
6 weeks PO	108 ± 18.8
Physical therapy required
No	61 (68.5)
Yes	28 (31.5)
MUA
No	88 (98.9)
Yes	1 (1.1)

Abbreviations: EBL, estimated blood loss; ISR, Insall-Salvati ratio; MUA, manipulation under anesthesia; PO, postoperatively; POD, postoperative day; PTS, patellar tendon shortening; ROM, range of motion; TXA, tranexamic acid.

Values given as mean ± SD or N (%).

Results

Of the 89 patients included in the final cohort, 54 (60.7%) patients comprised the significant PTS cohort while 35 (39.3%) patients comprised the insignificant PTS cohort ([Table 3]). Patients in the significant PTS cohort were more likely to have undergone right primary TKA; however, no significant demographic differences in patient age, sex, race, ethnicity, BMI, ASA or CCI score, and preoperative KOOS-JR and VR-12 scores were found to exist between cohorts.

Table 3 Demographics of all patients who underwent primary total knee arthroplasty by presence of significant patellar tendon shortening on postoperative day 0 (n = 89)

	Insignificant PTS	Significant PTS	P-value
	(n = 35)	(n = 54)
Age (years)	67.2 ± 7.2	66.7 ± 8.9	0.970
Sex			0.996
Man	11 (31.4)	17 (31.5)
Woman	24 (68.6)	37 (68.5)
Race			0.612
Black	9 (25.7)	15 (27.8)
Other	2 (5.7)	1 (1.9)
White	24 (68.6)	38 (70.4)
Ethnicity			0.250
Hispanic or Latino	0 (0.0)	2 (3.7)
Not Hispanic or Latino	35 (100)	52 (96.3)
Laterality			0.041
Left	20 (57.1)	19 (35.2)
Right	15 (42.9)	35 (64.8)
ASA	2.4 ± 0.5	2.5 ± 0.6	0.390
BMI	32.7 ± 5.8	33.2 ± 5.5	0.756
CCI	3.9 ± 1.9	3.9 ± 1.8	0.990
Preoperative KOOS-JR	41.4 ± 21.0	43.8 ± 21.0	0.275
Preoperative VR-12	30.4 ± 16.1	30.5 ± 15.3	0.953

Abbreviations: ASA, American Society of Anesthesiologist's physical status classification score; BMI, body mass index; CCI, age-adjusted Charlson comorbidity index; KOOS-JR, Knee Injury and Osteoarthritis Outcome Score for Joint Replacement; VR-12, Veterans Rand 12 Item Health Survey.

Note: Values given as mean ± SD or N (%).

Preoperatively, no significant differences in ISR, active flexion, extension, or total ROM were found to exist between cohorts ([Table 4]). Perioperatively, 33 (94.3%) patients in the insignificant PTS cohort and 51 (94.4%) patients in the significant PTS cohort received spinal anesthesia (P = 0.975). Cut to close time, EBL, tourniquet time and use, as well as TXA dosage and use were similar between cohorts. On POD 0, ISR was significantly less for patients in the significant PTS cohort at 0.9 ± 1.2 when compared to 1.1 ± 0.1 for patients in the insignificant PTS cohort (P < 0.001). When compared to preoperative imaging, patients in the significant PTS cohort experienced a decrease in their ISR by 21.9 ± 8.7% while patients in the insignificant PTS cohort experienced a decrease of 0.8 ± 10.9% (P < 0.001) on POD 0.

Table 4 Clinical characteristics of all patients who underwent primary total knee arthroplasty by presence of significant patellar tendon shortening on postoperative day 0 (n = 89)

	Insignificant PTS	Significant PTS	P-value
	(n = 35)	(n = 54)
Anesthesia			0.975
General	2 (5.7)	3 (5.6)
Spinal	33 (94.3)	51 (94.4)
Cut to close (min)	85.5 ± 11.6	83.4 ± 13.5	0.411
EBL (mL)	3.1 ± 7.3	7.1 ± 10.6	0.053
Tourniquet use
Yes	35 (100)	54 (100)
Time (min)	89.4 ± 17.0	81.9 ± 13.5	0.198
TXA use			0.261
No	12 (34.3)	23 (65.7)
Yes	25 (46.3)	29 (53.7)
Dose (mg)	960 ± 96.2	997 ± 210	0.670
ISR
Preoperatively	1.1 ± 0.2	1.2 ± 0.2	0.306
POD 0	1.1 ± 0.1	0.9 ± 0.2	<0.001
6 weeks PO	1.2 ± 0.2	1.1 ± 0.2	0.253
∆ ISR (%)
Preoperatively to POD 0	−0.8 ± 10.9	−21.9 ± 8.7	<0.001
POD 0 to 6 weeks PO	7.6 ± 12.0	25.0 ± 15.8	<0.001
Preoperatively to 6 weeks PO	5.7 ± 7.9	−2.9 ± 10.9	<0.001
Flexion (degrees)
Preoperatively	107 ± 14.2	106 ± 13.0	0.667
6 weeks PO	110 ± 15.0	110 ± 21.4	0.668
Extension (degrees)
Preoperatively	4.0 ± 4.4	4.6 ± 4.6	0.578
6 weeks PO	2.1 ± 3.7	1.8 ± 4.3	0.434
Total ROM (degrees)
Preoperatively	103 ± 16.8	100 ± 21.4	0.643
6 weeks PO	105 ± 24.0	110 ± 14.4	0.329
Physical therapy required			0.214
No	23 (65.7)	38 (70.4)
Yes	12 (34.3)	16 (29.6)
MUA			0.418
No	35 (100)	53 (98.2)
Yes	0 (0.0)	1 (1.1)

Abbreviations: EBL, estimated blood loss; ISR, Insall-Salvati ratio; MUA, manipulation under anesthesia; PO, postoperatively; POD, postoperative day; PTS, patellar tendon shortening; ROM, range of motion; TXA, tranexamic acid.

Note: Values given as mean ± SD or N (%).

From POD 0 to 6 weeks postoperatively, significant PTS patients experienced an increase in their ISR by 25.0 ± 15.8% while patients in the insignificant PTS cohort experienced an increase of 7.6 ± 12.0% (P < 0.001). When comparing change in ISR from preoperative imaging to 6 weeks postoperative imaging, ISR decreased by 2.9 ± 10.9% for patients in the significant PTS cohort and increased by 5.7 ± 7.9% for patients in the insignificant PTS cohort (P < 0.001) but at 6 weeks postoperatively, there was no significant difference in ISR between cohorts. Additionally, total range of motion, flexion, or extension was similar between groups at 6 weeks postoperatively. Similarly, 12 (34.3%) patients in the insignificant PTS cohort participated in formal, outpatient PT while 16 (29.6%) patients in the significant PTS cohort did (P = 0.214). One (1.1%) patient in the significant PTS cohort underwent MUA for knee stiffness while no patients in the insignificant PTS cohort required MUA (P = 0.418).

Discussion

In the present study, 54 (60.7%) patients underwent significant PTS following primary TKA while 35 (39.3%) patients did not. Patients with significant PTS were more likely to experience a decrease in their ISR immediately following TKA (−21.9 ± 8.7% versus −0.8 ± 10.9%, P < 0.001). Although by 6 weeks postoperatively, there were no significant differences between cohorts with regard to ISR, there were significant difference in ISR dynamics between groups; differences were seen between group from POD 0 to 6 weeks postoperatively (25.0 ± 15.8% versus 7.6 ± 12.0%, P < 0.001), and from preoperatively to 6 weeks postoperatively (−2.9 ± 10.9% versus 5.7 ± 7.9%, P < 0.001). ROM, PT needs, and MUA rates did not differ between groups. Although ISR changes occurred perioperatively, POD 0 PTS did not predict long-term PTS or patella baja.

Although primary TKA remains an extremely successful procedure for the definitive management of end-stage knee OA, complications following TKA that involve the patellofemoral joint remain common causes of revision TKA.[1] PTS, defined as a decrease of 10% or greater in ISR and a characteristic of patella baja, has been reported to range from 34 to 38% at 1 and 5 years postoperatively.[3] [4] However, the etiologies of PTS following TKA have yet to be fully understood; PTS in the acute, postoperative period may be the result of excessive tissue tensioning during arthrotomy closure. Ischemic injury secondary to tourniquet use, infrapatellar fat pad excision, and lateral retinacular release have also been postulated to contribute to chronic PTS.[3] [7] [8] [9] [10] Clinically, patients with PTS following TKA may describe anterior knee pain, crepitus, reduced ROM, and decreased PROMs, possibly necessitating prolonged PT, MUA, or ultimately revision TKA.[5] [6] To our knowledge, this is the first study to identify the incidence of significant PTS in the acute, postoperative period and to what degree postoperative patellar tendon lengthening occurs in patients during this time period.

The incidence of PTS following TKA has been reported in current literature to range from 12 to 38%.[3] [21] [22] Acute PTS following TKA has been proposed to result from aberrant soft tissue tensioning during surgical wound closure; however, Masri et al found no significant difference to exist in early ROM recovery and PT requirements when the capsular closure was done in flexion versus extension.[15] In our study, all TKAs were performed by the same surgeon to minimize the variability between cases in surgical technique with all knees closed similarly, step by step. However, 54 (60.7%) patients in our study were still found to have had significant PTS immediately following surgery, though there were no differences by 6 weeks post-surgery. Our findings, in combination with those of Masri et al, suggest additional etiologies of acute PTS to exist for these patients besides surgical closure technique. Alternatively, Noyes et al proposed decreased strength of the quadriceps femoris to cause contracture of the patellar tendon.[7] Our study did not include quadriceps strength as a direct outcome measure; however, active knee extension at 6 weeks postoperatively may serve as an indirect measurement for quadriceps strength since patients in both cohorts were able to extend their knees greater than preoperatively.

Although the exact cause of acute PTS following TKA remains unclear, no significant differences in 6-week ISR, active ROM, or PT requirements were observed between patients with and without early PTS. These findings suggest that patients with significant PTS were able to regain patellar tendon length without additional intervention. As suggested by Davies et al, preoperative pain and degeneration secondary to OA may lead to shortening of the tendon with correction occurring after surgery as function improves.[4] Our standard postoperative protocol for primary TKA patients involves a series of home stretching exercises that patients are taught prior to surgery and instructed to perform daily upon discharge; patients are not prescribed PT unless they are stiff at their 2-week postoperative clinic visit. Our protocol may not only benefit patients' patellar tendon length by limiting the extent to which their quadriceps decondition but may also help patients to regain their ROM and promote movement, reducing the acute inflammatory processes that may contribute to PTS following TKA.[11] [12]

Although this is the first prospective study to determine the incidence of significant PTS following primary TKA in the acute postoperative period and identify when patellar tendon lengthening occurs in these patients, it is not without several potential limitations. This was a prospective, cohort study performed at a single institution by one arthroplasty surgeon. Therefore, the generalizability of our findings may be limited to patients who undergo primary TKA using similar surgical techniques and do not allow for comparisons to be made between alternative approaches and techniques. Additionally, the overall sample size of the present study was relatively small. However, the power analysis conducted prior to the start of our study indicated that 89 patients were adequate to power our study and appropriately identify differences between cohorts. It is also possible that no interventions such as manipulation under anesthesia (MUA) were required, thus skewing our findings. Lastly, despite having a minimum of 6-week clinical and radiographic follow-up for all patients, our follow-up time is too short to characterize the potential long-term impact of acute PTS on PROMs.

Conclusion

Based on the findings of our study, majority of patients undergoing primary TKA will experience significant PTS in the acute, postoperative period. However, we found that by 6 weeks postoperatively, these patients will appreciate patellar tendon lengthening and resolution of acute PTS without requiring additional PT or experiencing deficits in ROM.

Conflict of Interest

The authors declare that they have no conflict of interest.

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Publication History

Received: 25 April 2025

Accepted: 06 November 2025

Accepted Manuscript online:
12 November 2025

Article published online:
01 December 2025