Healthy knees have a highly variable patellofemoral alignment: a systematic review

Key points

1. 01Across 15 studies of healthy non-arthritic knees, all six common patellofemoral alignment measures showed wide variability.
2. 02Normal ranges overlapped heavily between studies, blurring the line between healthy and pathological.
3. 03Differences in imaging method (weight-bearing versus non-weight-bearing) and measurement technique added to the variation.
4. 04The authors call for standardized imaging and measurement protocols before alignment thresholds can be trusted.
5. 05Using one cut-off value to decide on surgery or rehabilitation may be misleading given how much healthy knees differ.

How it was conducted

Design

Systematic review (PRISMA) of MEDLINE/EMBASE through Jan 11, 2019

Participants

Healthy non-osteoarthritic knees without instability, ages 15 to 47 years

Studies included

15 studies, English or German, excluding instability, osteoarthritis, n under 10, and non-standard measures

Parameters assessed

Sulcus angle (SA), femorotrochlear depth (FTD), patellar tilt angle (PTA), lateral patellofemoral angle (LPFA), lateral femorotrochlear inclination (LFTI), and TT-TG distance

What they found

• Sulcus angle (SA) ranged from 118.7 degrees plus or minus 7 to 168 degrees across studies.
• Femorotrochlear depth (FTD) ranged from 3.4 plus or minus 1.1 mm to 7.1 plus or minus 1.8 mm.
• Patellar tilt angle (PTA) ranged from 0.7 degrees plus or minus 4.99 to 17.05 degrees plus or minus 4.3.
• Lateral patellofemoral angle (LPFA) ranged from 6.26 degrees plus or minus 4.1 to 11.1 degrees plus or minus 4.0.
• Lateral femorotrochlear inclination (LFTI) ranged from 16.3 degrees plus or minus 2.8 to 22.1 degrees plus or minus 1.9.
• TT-TG distance ranged from 9.8 plus or minus 4.6 mm to 17.3 plus or minus 5.3 mm.

Limitations

• Wide inter-study and intra-population variability with no pooled meta-analysis, limiting precise normal reference ranges.
• Differences in imaging (weight-bearing versus non-weight-bearing) and measurement technique between studies make direct comparison difficult.
• Restricted to English and German studies and to ages 15 to 47 years, which may limit generalizability.
• Number of healthy knees studied per parameter and overall sample size are not reported in the available text.

Why it matters

For patients

A single kneecap alignment measurement on your scan may look unusual yet still be perfectly normal, so it should be interpreted alongside your symptoms, not in isolation.

For clinicians

Avoid relying on one alignment threshold to define patellofemoral pathology or to justify surgery, since healthy knees span very wide and overlapping ranges.

For readers

Normal patellofemoral anatomy is far more variable than fixed cut-off values suggest, underscoring the need for standardized imaging and measurement protocols.